forecasting - modifying an forecast

See the attached spreadsheet:

I am attempting to forecast January numbers. Currently, the numbers in bold/yellow

If the previous month (December) has a value over 200, add 15 to the forecast in B14 (January estimated).
If the previous month (December) has a value less than 200, subtract 15 from the forecast in B14 (January estimated).

In the attached example 290.66 would have 15 added to make the total of 305.66.

Thank you!