If you're doing it yourself don't you also (technically) have the problem of disposing of anything still left in the system, as well? Of course there's probably nothing left in there..
IF there is anything left in the system (checked with said gauge ) you probably don't have much of a leak ,top up ,job done
IF the system is empty , it will be full of air ,and moisture , which needs removing ,along with replacing the air dryer, and fixing the cause of the leak.
surely better than squirting cans of stop leak and r134a in that will just leak out again in a leaky system full of moisture ,
Unless your going to be doing air-con on a few cars, it's better using and paying someone to connect it up to a dalek,, r134a will cost upwards of £200 for a cylinder. Most air-con problems are condenser, leaking. Omega condenser can be had for £@70.
small cans of r134a available , you don't have to buy a big cylinder ,
As Mr 456lbft has paid quite a bit for his Omega , I expect he'll be keeping it long term ,so investing in the tools to resolve air con issues may pay dividends long term.
taking it to an "air conman"
will probably involve a few costly trips ,check system,order parts,fit,check,fill
a system empty can be checked under pressure with air to find leaks ,check the electrics are all working (only briefly) etc etc ,plus a long overnight check to see what drop in pressure occurs (not a 15 minute vac check on a darlek) , then vac out ALL moisture ,fix issues , then when the system is gas tight ,put R134a in , which has to be better than releasing loads of gas into mother earth , not that i'm a tree hugger