Search the Internet for 'the 4psi rule' - it's been used (& published) by all sorts of people including tire manufacturers & tire users of all types; it's been around almost as long as pneumatic tires have been around; it works for any road surfaces & under any conditions for any type of vehicle (that's running on pneumatic tires!); and it's a whole lot simpler to use than it is to explain! While some use 10% & some use 'footprint area' and others use tread temperatures, done properly they all end up with close to the same pressures, but the easiest & simplest 'rule of thumb' that I was taught as a very young fella in the tire design & test world & the method I've seen used most often & most widely, including by some very well established & respected tire manufacturers as well as widely across the whole spectrum of activities & industries that use pneumatic tires, is to aim for a 4psi increase after an hour's driving, hence 'the 4psi rule'.
To achieve this, check the cold tire pressures before you start; then drive for an hour (ish!) Check the tire pressures again as soon as you stop! Ideally, the 'hot' pressure will be around 4psi above whatever you used & checked as your cold start pressure, because the tire should have warmed up thru use and that'll be reflected in a pressure increase of the air inside the tire. If your tire pressures have gone up by MORE than 4psi, your cold start pressure was too LOW, cos the tires have been too soft for what your doing & have flexed too much; while if the pressure has gone up by LESS than 4psi, your cold start pressure was too HIGH and your tires haven't flexed enough to warm them properly.... which means you'll need to adjust your cold start pressures as indicated by the pressure increase.
To allow for ambient temp air going into the already hot air inside the tire, you'll need to adjust the air pressure in your tires up or down as indicated by the 'rule' using only ONE HALF of the pressure difference between what your hot pressure reading
IS & what it
SHOULD HAVE BEEN if it had gone up by 4psi. Use the nearest whole number with this - there's no point in trying to adjust by 1/2 a psi, the tires on most 'passenger' or recreational vehicles are too small to make an appreciable difference with such a small change & the pressure gauges we tend to use are rarely accurate enough or graduated finely enough to make that small a difference usable anyway!
So, if your tire pressure started out at 18psi and after an hour or so's riding they only reached 19psi, then the cold start pressure was too high and you need to drop your pressure by 1/2 the difference between 19 (what it IS) and 22 (what it SHOULD have been if it had gone up by 4psi)... 22 minus 19 = 3, so you need to drop 1/2 that, which is 1.5 psi - except you probably won't be able to read that accurately on your gauge or be able to achieve that drop, so I'd aim for dropping 1 or 2 psi rather than 1.5psi. But when you start riding on cold tires next time, remember that 18 was too high, so maybe start out on 16 or 17 psi for your next ride & repeat the check after an hour's ride to see how close they are this time.
However, if you started at 18 psi & when you checked again after an hour or so's riding, your pressures had increased to 28 psi, then your cold start presssure was too LOW & you need to ADD air now! One half of the difference between 28 (what the pressure IS now) & 22 (what it SHOULD have been) works out at 3 psi ie 28 minus 22 = 6, & 1/2 of 6 = 3..... so you should ADD 3psi before you ride on & for your next ride you should remember that 18 was too low so you need to start your next ride at saay 21 psi cold & then check the pressure in your tires again after an hour's riding.
Do that 'Check, ride, check, adjust' thing often enough & follow the 4psi rule, then every time you'll be getting closer to YOUR optimal pressure/s for the given tires under the given load, driving style, & conditions. Sure, changes in any of those variables like ambient & road surface temps, the load on your bike, or how fast & where you are actually riding will probably cause some small variations in the results for a given ride, but you should fairly quickly learn to recognise the different pressures that will best suit a given ride subject to the changes in any of those variables that you might reasonably expect. Hot days &/or high speeds generally need a little more pressure than normal; while cold days &/or slow speeds and especially wet roads generally need a little less pressure to get that ideal pressure increase & the best from your tires. But, however long you take to learn or how often you actually apply the 'rule', every time you do apply it you'll be helping yourself to run the ideal pressure for what you are doing every ride and that ultimately means you that are more likely to get the best from your tires! :thumbup:
Sorry about the length of this post, but I did say it's harder to explain than it is to actually do!! :shocked:
hyea:
Your choice; but others might like to do otherwise.... 
