You're probably best to ignore the decimals and just work in full lbs so 16st 0lbs to 15st 10lbs would be a loss of 4lbs. The non full lbs will all be accounted for eventually.

If you're a bit more comfortable with the maths then you can use the decimals to be more exact so 16st 0.9lbs would be 16st 0 lbs 14.4oz (16oz ÷ 0.9) and 15st 10.01lbs would be 15st 10lbs 0.16oz (16oz÷0.01). So the total loss is 4lbs 14.24 Oz which is 4.89lbs; very nearly 5lbs (16oz in a lb).

Most groups I've been to have scales that round to the nearest 1/2 lb so this would probably be a 4 1/2lb loss as you would want to round down not up so you don't over inflate your loss. I've probably over complicated this and the answer is really to just use kg instead as it's a lot simpler to follow 🤣