[hcobb]

Just have each level of toughness negate one point of Armor -DX and one point of Armor -MA.

You can buy infinite levels of toughness, but the ST requirement scales by the size of your figure.

Man sized or smaller: first level at ST 12 then +2 ST thereafter.
Large One hex figure: first level at ST 15 then +3 ST thereafter.
Two hex figure: first level at ST 16 then +4 ST thereafter.
Three hex figure: first level at ST 18 then +6 ST thereafter.

And so on. Each level of toughness costs only two memory points.

So a ST 18 human pays 8 memory points for four levels of toughness and is then at full MA and no DX reduction for Fine Plate armor, with a total of 10 hits per attack ignored. (A ST 24 Reptile Person could do the same, or a ST 36 Giant.)