Is it safe enough to do this without a manifold gauge and only with the gauge on the bottle from Autozone?

Currently the system has absolutely no pressure.

 

Unless it's leaking at the evaporator under the dash, u may be able to see an oily residue on a/c lines or components under the hood. That's a good indication of a refrigerant/compressor oil leak.

 

That is the problem, I don't notice any leaking. It must have happened in the last week, Florida got some nasty cold temps (25-30degs) and there was no need for A/C until yesterday.

I filled it with some refrigerant with lubrication and a second can of straight r134a to 30psi. I know that it's supposed to be 35, but I don't trust that gauge.

I'll see if it leaks again during our next cold snap on Friday.