2009 AM Q9 Bii

I’ve used search but I can’t find the answer I’m looking for. In the solution to Q9 Part Bii for calculating the credit risk of the JPY put, they use the formula: 1/100 -1/102.5 = 0.000244 x contract size x no. of puts where 100 is the strike rate 102.5 is the spot rate where does this formula come from? I can’t see it in reading 40 anywhere. Is it a part of LII I have blanked out?

it is mentioned in derivates. forget which reading but i believe ss 15.

thanks junkie, don’t have the books at hand but will check