The 41822 isn't suitable for a 1275. It advances too fast and has too much total advance. You need to change the stop (or fit a hard shim) as well as the springs to adjust this.
Really, unless you are being supplied a specific kit that will achieve a specific outcome (eg a Lucas 41768 Kit) and without a Distributor Dyno, anything else is a guess. You can do it 'in car' with a dial back timing light and loads of time, but is not an ideal way to check the effect of any modifications as there's so many more items in the drive train between the crank (where you would be measuring it) and the distributor.
If you are after a Distributor to suit a stock 1275 engine, Mini Spares have them complete ready to fit, they are actually right (unlike so many others I've tested) and I feel, very fairly priced.
When going to a modified engine, really, the engine needs to be run up on a dyno to see what curve is needed, though many companies like H&H or AC (on the forum) will get them close enough if they know exactly what your engine spec is.