Welcome!
My go to load for mine is now 7.0g Green Dot under 250g RNFP 917fps. Was shooting 20g of IMR 4227 under 255g SWC at 1030fps which is a good load too. Just uses way to much powder for general shooting. 11.0g of AA#5 under 255g SWC at 961fps is another good one... 8.5g of Unique/Universal under 250-255g bullet for around 980fps.
If you want to dirty up your gun 23.0g (by weight) of Triple-7 under 250g bullet at 919fps is a good load too!
Another reference:
Brian Pierce .45 Colt Loads PDF . Oh, the New Vaquero referenced in the article is the same as your flattop... So you are good for Tier 2 loads.
You mean you bought one of these :
Really enjoy mine. Enjoy!!!
I get my bullets from Rimrock bullets. They are just right for hardness. I don't get any significant leading.