This is one of the problems with converting to 134, excessive high side pressure. On converted systems the high side pressure can sometimes go up to 300 PSI. The important thing is that you don't get into a situation where the pressure runs away. The high side pressure should stablize once the vent temp is stabilized. There are no hard and fast pressur or temp readings to look for. If you are getting a stable 40 degree vent temp and the high side does not continue to increase after stabilized, you are probably okay. Proceed cautiously when adding 134. This is another reason for staying with R12 in an R12 system.

Good luck,