Go ahead and add 134a. I just went through the same exact thing with my '97 850. Had the fast cycle thing happening, and very poor A/C performance. To my surprise, the pressure reading was 25-45 psi (i.e. normal). I decided to add 134a anyway, and the cycling stopped while I was adding it. The pressure reading did not change at all though, even though I must have added about 8 oz! A/C works fine again now.
I've read that the pressure readings are not a reliable measure of the coolant fill level anyway. The only way to be sure you've got the right amount, is to get all of the coolant out of the system, then refill it with the specified amount. But that's for the pros. For DIY purposes, I would just add coolant to the point where the fast cycle stops and the air blows cold, then stop. Overfilling must be avoided.