Amount invested (90,000) multiply by interest rate and divide by 1200 to get gross monthly interest. This is an indication only, as most investments have interest calculated on a daily basis, and each month has a different number of days. (e.g. Feb=28, August=31 days)
e.g. 90,000 x 3.5 (guesstimate per annum interest rate) divide by 1200 = 262.50 average monthly gross before tax
Alternatively and more precisely - amount invested (90,000) multiply by interest rate, divide by 36500 and multiply by number of days in that month.
E.g. 90,000 x 3.5 (guesstimate per annum interest rate) divide by 36500 and multiply for 28 days (Feb) = 241.64 gross (Before income tax)