You need to inflate it to just over 100 so that the air that escapes when you pull the inflater leaves the tire at 100.
An easy trick is to press in on the tire with your shoulder as you're inflating; really push into it, and it adds just enough to offset the air loss when you stop inflating. Dad used to use his head, and that worked great because the smaller surface area of the top of his head meant more PSI, but I'm not as tough as dad and it hurts my neck to do it that way, so I use my shoulder.
Anyway, don't give up! You can get there, and a properly inflated tire can improve your mileage. Most people don't keep their tires properly inflated and are just wasting money.
Jesus Christ! A friend of mine exploded a mountain bike tire when trying to inflate it to 100psi, assuming it would be the same as on his racing bike, he said it went off like a grenade. I can't imagine what a car tire would be like.
I've got one better... I used to work in a bike shop. When building new bikes, it's common to overinflate the tires (to get the bead seated nicely all around) and then let the excess out. I did that one day with a flat repair, and blew the side of the rim off, it had been worn so thin by the brakes. (This was many years ago, before disc brakes were common)
Also, we had a deaf girl working with us. If someone exploded a tire (it didn't happen often, but it was more than never) it would startle her too, she didn't hear anything, but she said she could feel it.
I've done this with a brand new tub, slipped between the tire and the rim and I just missed it, and it happens quick. Not exactly a grenade, only know that from having been near them too, but loud enough to snap your head real fast.
Just explain to them they overlooked it is in psi and not % and all parties will be safer. It was probably a joke by the creator and by you.. but ignorance isnt bad, it's an opportunity for improvement. -Brought to you by Hallmark