A millibitcoin - MBTC or MBT for short is simply 1 one thousand of a bitcoin. So if bitcoin is worth 10,000 EUR a MBT is worth 10.

The easiest math to do is simply think of it as part of a BTC .  

For example 

520 MBT then is actually  0.520 Bitcoin

We do this to avoid rounding errors. Even at six decimal places you may be losing money if you don’t use the MBT is conversion.

Why does Tempo use MBT MBTC or millibitcoin instead of full bitcoins in its calculations?

Very simply - it makes bitcoin calculations more accurate and ensures that you or we don’t loose money.