The argument one gets is guys say that the directions on these KITS use just the low side reading , so that should be fine...BUT, on our systems , we are dealing with an AUX FAN that has SPECIFIC specs for proper operation and that means we need an accuraate charge for all systems to work in conjuction of one another ..you wind up with no low fan b/c you are short just a couple of ounces and you can easily wind up with an over-heating engine temp.
The correct way is to evacuate and recharge by capacity spec, but both gauges for topping is fine , as is eyesight if need be..certainly not low side reading only.

These low side only KITS are for generic a/c systems that do not have to contend with aux fan cut-in specs and balances of system pressures as Benz uses.