I am modelling a one time contribution payment with this conditions:

1 it happens only after x months (in this case 36, but need to build the model so these number can change). I have set a column that counts down until 0, which is the month the payment has to be done.That way the payment is always done when the value in that column is 0.

2 There is a condition, that the payment will be done only a certain number of times (in this case only once after the first 36 months) but the model needs flexibility, so it can happen two or three times after 36, 72, 108 and so on.

Any suggestions?