What is "margin percentage in metatrader4"?

Hi
Been a while since I’ve been here, I like what you’ve done with the place!!

I’m trying to get an IRON grip on how margin works for different pairs.

I want to be able to calculate EXACTLY how much margin is required for any given forex pair within my broker’s MT4.

My real question here is WHAT THE HELL is Margin Percentage in the MT4 Market Watch ‘contract specification’ tab?
(Right-click any pair in the Market Watch pane, and select ‘Specification’)

I’ve checked it for all 28 major and minor pairs:

EURUSD 50%
SP500 0. 5%

Thanks