It starts here -
You said you are a high earner, but high is relative. This tax table will show what marginal rate you have for your taxable income. Look at your 2015 return to get a better idea what "taxable" means, vs gross income. For starters, with a standard deduction, and just the two of you, $20K comes off your gross. Even so, let's assume you are in the marginal 25% bracket.
The W4 form does offer instructions on how to calculate how much extra to withhold, but, to your point (and brilliant criticism of the process) withholding is not available as a percent, only as normal withholding, i.e. if spouse's income were a flat, predictable number, or as an extra, fixed, number, per check.
You shared with us in your other question she expects to earn $7K-$15K. The average of these 2 numbers is $11K, which at 25%, is $2750. I'd divide that over the number of checks remaining this year (20?) and just withhold that much extra. Use your tax software from last year or an online calculator and in 3 months, see if you are on track. You can adjust the W4 any time to get as close to the actual total year tax due as you'd like. My answer focuses on the 'adjust repeatedly,' part of keshlam's own answer. His quarterly payments suggestion also works and you might prefer it. In general, mine would only take 1-2 adjustments per year at most.
If you withhold too much, as most people seem to do, you'll get it back when you file. But, worst case, you withhold $3750 and she makes just $8000. This is $1750 too much. The average refund is over $3000. Too little, and mhoran's answer explains when you'd owe a penalty.