Well, I was basically thinking like this:
Mining potential would be the total amount of minerals a given planet could dig in a year, let's say that we're at 300 mines with a mining rate of 10 tons per year per mine, so 3000 tons in total.
Weighted share of minerals by availability would go, for example, go with Duranium (0.7), Gallicite (0.2) and Mercassium (0.1). Total availability is 1.0, total mining rate is 3000 tons, Duranium has 7 shares of the mining rate (total mined in a year 2100 tons), Gallicite has 2 shares (total mined 600 tons) and Mercassium has 1 share (total mined 300 tons). Total mined is 3000 tons of material.
Weighted share with loss of potential from lack of availability would go 0.7 Duranium, 0.2 Gallicite, 0.1 Mercassium, 0.3 unavailable, 0.8 unavailable, 0.9 unavailable, total 2.0 unavailable, and mine 700 tons of Duranium, 200 tons of Gallicite and 100 tons of Mercassium. The remaining 2000 ton capacity is 'wasted', dredging up Newtonian materials from the depths of the aether that are not relevant to the game or otherwise just expended effort that is not useful. There would still be 3000 tons of material mined in total even 2/3rd of it is discarded.
Being able to turn certain minerals off in a mining operation is helpful in this case because it lets you go for 0.7 Duranium and ignore the impact and loss of efficiency due to low availability of other materials, you'd still need to account for the 0.3 unavailability rating though. So in that case you'd be mining 2100 tons of Duranium, 900 tons of discarded waste material, and not touch the Gallicite and Mercassium deposits.
Being able to turn off certain minerals in a place where you are always mining the maximum possible of the mineral wealth of the planet with the availability ratings only impacting ratios of the materials dug up would kinda break the system because it'd allow you to use this setup to mine 3000 tons of material from an availability 0.1 material from a planet with only one or a few materials, and would render a body with all minerals at 1.0 availability but in small quantities strictly inferior to a body with vast quantities of all materials at only 0.1 availability simply because that body is going to be mined for a long time to come.