[GRITTY REALISM]Proposed House Rule Modifying Skill Maintenance

[lugaid]

In another thread, RyanW proposed the following modification to the Maintaining Skills optional rule on p. 294 of the Basic Set:

Originally Posted by RyanW

Like most things, I would instinctively use the SSR scale. Something like:

skill  hours/week
att+5  1
att+6  1.5
att+7  2
att+8  3
att+9  5
att+10 7
att+11 10
att+12 15

At att+17, you're basically spending every waking moment practicing.

I liked the idea and adopted it into my set of "Gritty Realism House Rules", but developed it slightly, as follows:

Maintaining Skill (p. 294) is adjusted to require the following number of hours per week of practice for higher Skill levels:

Skill	Hours/week
att+5	1
att+6	1.5
att+7	2
att+8	3
att+9	5
att+10	7
att+11	10
att+12	15
att+12	20
att+13	30
att+14	50
att+15	70
att+16	100
att+17	150
att+18	200

Beyond att+17 is generally impossible without extensive field use, as there is not enough time in a week to maintain such a high level of Skill. In a practical sense, att+16 or att+17 are probably unlikely without Advantages like Doesn't Sleep. Alternately, use in the field on a given day counts for 1/5 of the required hours for that week (round fractional hours down, but for levels below att+9, field use counts as the entire week's worth of practice time for that Skill). Practice, including notional "practice" from field use at higher Skill levels, beyond the amount required to maintain the Skill counts as Self-Study. For this purpose, "field use" is defined as the GM asking the player to make a roll using that Skill in an adventuring context, regardless of the success or failure of that roll.

Am I missing anything obvious that would make this unworkable (other than the slight addition to record-keeping, which I don't consider particularly onerous)?