Trying to make simple calculations in Power Pivot

Hi Folks,

Rank Newbie with Power Pivot and Dax.

I have to have 400,000 baht in my Krungsi Bank Account by October 1. I created a very simple table that showed where I was in January and fed that to the Data Model. The most important number i need is how much I need to put into that account each month - for all intents a running balance. Could someone have a look? If things arent described well enough, please let me know.

The sheets of interest in Power Pivot are "dkrung", and in the regular workbook sheet "Krungsi Bank Amounts"

I know I have a lot to learn, and i dont expect people to spoon feed me the answer, but some general guidance like "go study calculated fileds and measures and relationships" is fine.