Month on Month Growth and Rev.

I am trying to project that part of the new users that are gained in our business will use certain services, spend some money and help in generating revenue, and also part of the existing users will do the same.

the current assumptions I am working with are that ... there is a month-on-month growth with the userbase ...

if month one has 100 users and month two has 120 users I am assuming that 120-100*15%*3 = 15% of new users will spend about 3$ ... and 5% of existing users will do the same ...

this is the thought process behind it not sure if I have explained it well enough.


I have attached data set where monthly users are growing ...
to calculate the revenue I want to show the following assumption ...

10% of new users every month or bringing $3 each
and 5% of existing users or bringing in 3$ each

I have so far done a simple formula but was wondering if there's a better way to do it which is more accurate on projections.