You should not be using a set of R12 gauges on 134. Cross contamination is NOT healthy for either system. Converting gauges is a very bad idea.
The pressures on the gauges are the same however. Pressure is pressure.
There is no exact proper pressure for a converted system. You will have to experiment a little to get it so it will cool well.
There are many here who think it's not a good idea to convert a 124 car to 134. It's not because it's a bad idea, it's just that some cars convert well and some don't. The 124 car is one that does NOT convert well. That said, if you don't live in Houston or Miami, you might get by with it just fine. Also, if you're driving is mostly constant speed highway driving it might also work for you.
The important thing in a conversion is that you evacuate the system and use oil for 134. The death kits that you buy at Wal Mart or Pep Boys give instructions for not evacuating. This approach does not have the "death" connotation attached to it for nothing. It will eventually kill your a/c system. If you want to convert, you should, at a minimum, replace the filter drier, put in 6 oz. of Ester oil and evacuate. If you do that, you will have a reasonable chance of success, it may not cool as well as R12, but at least you won't kill the system. Cross contaminating with the R12 gauges will do nothing for ensuring your success either. A 134 gauge set and connectors can be had for about $100 or less now. That's peanuts compared to the cost of all the components that you stand to ruin.
Sorry for the lecture, I just want you to be successful.
Best of luck,