Position Sizing with XAUUSD, XAGUSD and WTI

How do we work out position sizing with XAUUSD, XAGUSD and WTI?
I like to use a fixed dollar amount for my risk.
What I do to work out my position sizing when I trade fx pairs is:

Say I wanted to risk $50 on a trade and my stop loss distance is 72pips, I would:
Risk amount / Stop Loss number of pips
50/72=0.69
Then enter 0.06 in MT4 which gives 0.60/pip.

I don't practice the % risk model, only the fixed dollar amount risk ie I know how much I am going to risk on a trade and I know the number of pips before entering as well.
However I could use a formula for position size that includes % risk as I will also know the % of my account that I will be risking on the trade by (risk * 100) / total capital in account.