there is a lot of bad information here. First off just because a tire says the max psi is 90 or 110 or whatever doesn't mean you need to use that much air, it's a lot like car tires, the psi recommended on cars varies from car to car, and it's the car manufacture that determines the correct psi for each model...not the tire manufacture, the same is true with bicycle tires except the bicycle manufacture has no clue as to how much weight will be on their bikes...but neither does the tire manufacture, so you have to figure how to calculate that.
Now up until recently when wider aero rims came out this was pretty simple to calculate, thank God that today we have on the internet PSI calculators that take all the hard work out. Now these calculators are based on the old tried and true 15% tire deflection; so see this and I will explain what to do, this is for road bike tires:
On this site there are 3 calculators, you only need to concern yourselves with the middle (or second) one. You need to know how much you weigh with your cycling clothes on, and how much your bike weighs fully equipped with even full water bottles, you add those two up and enter the total where it says rider+bike. Then you go to the next box that says 40%/60% (this setting is more for someone either on a touring bike with most of the weight on the rear or riding a bike with a more sit up straight posture), click on the arrow and change that to 45%/55%; moving down to the next box that says front tire width, click on it and select your width of tire and do the same for the rear tire width; when you enter the rear tire width the calculator will automatically display your ideal PSI. Several notes on this. There is no setting for a 27" tire, but 27" is not that much different than a 700c so don't sweat that, secondly all you need to do is to convert your 1 1/4 (or 1 1/8) to MM which for 1 1/4 would be 32mm. Another note, this is for your average everyday street riding, if you are riding in the rain you can drop your psi by 10 on each tire. Another note, if you put in your total weight and find out that the calculator has suggested a psi greater than that on your sidewall then that tells you that you are riding on a tire too narrow for your combined weight, so go up one size and see what the calculator says.
For those of you with MTBs there is this calculator: http://mtb.ubiqyou.com/
On any of these calculators if the tire manufacture gives you a chart for weight and psi follow their chart not the calculator. Also if you live on very rough roads dropping your psi by 5 will help smooth out the ride a bit, also switching to the next size up tire and reducing your psi according to the calculator results for that size tire will help big time. If by some slim chance you ride on very smooth streets you can increase your PSI by 5 and get good results. Of course the calculator is simply a starting point, but while it's accurate for max tire wear and handling, some people want a smoother ride and some people want a firmer ride, so those may adjust their psi accordingly, but the calculator results are pretty much right on the money.
I have not found a calculator yet for the newer wider rims that allow the tires to run at lower psi, so not sure how to calculate that plus not sure if the old 15% sidewall deflection rule is valid for these new rims. As a guess I would say follow the calculator and reduce your psi by 10 should be ok.