Gordon, just as an FYI, NLA/ Stoddard make this for the 356 porsche guys... same concept, but made to be installed on the gauge end of business. Not sure what size resistor they use. 25$ plus shipping.
yours is a BARGAIN!!!
KUDOS to your wife for figuring the math out.,, I took the trial and error approach last summer. (one of the local radio sharks was going out of business so I purchased a bunch of capacitors/resistors on sale and spent a Sunday playing around!)