Index MT4 SL and TP

Hello Traders.
Does anyone know how to calculate the S.L and T.P for indexes on the MT4 platform? Apart from Forex, I love to trade S&P500 and SPI200, but I’m not entirely sure how to calculate my S.L and T.P. My calculations never match what MT4 has calculated as my S.L or T.P value.

The SPI200 has a contract size of 25. Usually I work out the difference between entry and my stop and convert the points to pips, then I divide it by the volume size and times it by the contract size. Sometimes it works.

But, If I only want to risk $100.00, how do I work this out? Do I have to convert points to pips, and how do I work out what volume to enter?

Many thanks

Samantha