It’s 3m for me every time , the sheets from w/s heaven will give you 3 sheets at approx 73 mm each - these cost around 4.50 each , you can also get a sample set for around £10 or £12 so you can try them 1st , backwards strokes for the lapping film work best but I find the abrasive type are ok with a forward and backwards motion. A decent paper cutter will give you perfect results when cutting the sheets to size ( just my own personal choice that works for me )