The razor blade idea works well if you are very careful.

Wait until it is fully cured.

Go down to 400 or 600 but use a very gentle touch and add a drop or 3 of soap to the water. Don't let the paint wad up and keep rinsing often. Work your way up to 1200 or 1500 if you can find it. Then go to rubbing compound and polishing compound. By the time you wax it you will have a much smother and glossy surface than even a well-sprayed non-orangepeel natural top paint layer.

If you must tape and can't get the hang of feathering go all the way down to a lug and paint the entire section. If you mask at the lug the paint ridge will be impossible to see. Either that or put the mask exactly where you plan on putting a cable clip/stop or other item on the bike like the front derailleur clamp. Sometimes you can paint all the way up to a decal and use that as your mask point but be careful because tape can cause a decal to peel -avoid putting even the high-end blue tape over old decals as it'll peel the color right out of them.