If my memory is correct the formula for calculating the value of resistors in parallel is the sum of the inverses equals the inverse of the sum. That sounds like double-talk.
Or to caluculate to get the require conversion value it would be the inverse of the target minus the inverse of the fixed value equals the inverse of the required conversion value.
I don't know if that makes sense, I'm finding this difficult to put into text...
1/30 - 1/90 = 1/(needed value)
or 1/(1/30 - 1/90) = needed value
My math says it needs to be 45 ohms, which is not a standard value, but 47 ohms is and would get you real close. Using a 50 ohm variable resister would allow you to adjust until it was perfect.
