Thanks @BritishLibrary and @RichardL, I tried out the Forecast function in google sheets but it seems to lack the so called ‘seasonality’ that the excel forecast.ets function can do. So I pulled my data temporarily into excel, ran the forecast data sheet, then pulled the results back into google sheets. I think I’ve now got what I was looking for;
I can now compare my current balance to this forecast and see whether I’m ahead or behind which will be really useful at a glance. I could even set up an IFTTT alert if my balance goes a certain amount under this forecast.