Variable Enhancements with Takes Extra Time?

[Trachmyr]

I wish to build an ability that allows it's effects to grow, the longer the user concentrates. The actual ability I want to modify is Snatcher (Permanent, Creation, and more). The longer the concentration time, the more levels of More Weight Enhancement can be used. It's important that the effects grow with time (consider a growing forest), so every doubling of time increases mass by 10, but if concentration is interrupted, what has already been created will remain... it's not an all or nothing effect. However, this could be used for a slew of other effects on different abilities... increasing area of effect, duration, range, or any other scalable quality.

What would be the best way to model this? I'm considering using Selective, then adding a slew of “More Weight” enhancements limited by increasing amounts of “Takes Extra Time”. Such as:

Large Items +50%
More Weight +50%
Selectivity +10%
More Weight +30% (Takes Extra Time -10%) [+27%]
More Weight +30% (Takes Extra Time -20%) [+24%]
More Weight +30% (Takes Extra Time -30%) [+21%]
More Weight +30% (Takes Extra Time -40%) [+18%]
More Weight +30% (Takes Extra Time -50%) [+15%]
More Weight +30% (Takes Extra Time -60%) [+12%]
More Weight +30% (Takes Extra Time -70%) [+09%]
More Weight +30% (Takes Extra Time -80%) [+06%]
Total: +242%

This allows 200lbs with 10 seconds of concentration, 1 ton with 20 seconds, 10 tons with 40 seconds... ...1 Million Tons with 21min20sec of concentration, 10 Million Tons with 42min40sec of concentration.

Not bad, but problems (no discount for reduced effect) occur if I wish to go above this level... or if I add on additional Limitations such as Cost Fatigue. This is aggravated if I wish to make it so lesser creation take less than 10 seconds... Also, would this be adequate to cover the “effect grows with time” aspect, or does it need something else to avoid the “all or nothing” problem?

What would be the best way to model this?