I believe your original cap was marked 100, but this pressure (14.5psi) may be hard to locate. The 100 refers to the percentage of a bar of pressure (14.5psi) used as a rating. Most caps I've seen recently are 120 or 140, i.e 120% or 140% of a bar, and this can be too high on a car with an old heater core. Do some shopping and get the 100 cap. In a pinch you can simply use a common old Stant cap with a 14.5-15psi rating.

Regarding your fill procedure, there should be a pronounced flat area visible in the OEM tank top somewhat below but near the neck. This is the fill line. Above this mark leads to the piddle drips through the overflow tube. A recovery system is helpful to purge air and raise efficiency of the whole system. Any parts house will have something that will fit. Take the new 100 cap and add the sealing gasket from the recovery kit so that there is a positive seal at the cap. Fill the radiator to the top, add coolant to the recovery tank and run through several drive cycles allowing it to cool overnight. At the end of the week you should have a purged system and a slightly lower level on the recovery bottle. Refill it to the cold line and you are good to go.