If you want a guess, mine would be about 61°F.

Ford

Per Reinhart, each Δ17℉ results in a doubling or halving of the proof time, a geometric progression.   Take the ratio of existing time to desired time (12 ÷ 19 = .63), multiply by .5, and take the square root (0.56, the geometric mean).  Multiply that by 17℉, and you get the amount of change required to change the rise time.  And the answer is 9.5℉, or there about. :)  So Ford would win the cigar.

cheers,

gary

A train leaves Chicago traveling west at 70 miles per hour...

...what are the nationalities of the conductor and engineer?

This mathematical precision thinking reminds me of the time, when my late husband, a flight test engineer, scolded me for unscientifically hanging up the unhemmed curtains first and then adjusting the hem by eyeballing. He laid the curtain carefully flat on the floor, measured the desired length exactly with a tape measure, folded the hem very precisely, attached it with pins, and then went to hang it up, very satisfied with his work.

You should have seen his face when the curtain hem's distance from the floor turned out to be totally uneven - the superior technical brain had not considered the quirks of an old house, where nothing is precise or level...

Karin

@ Paul:  When someone says, "It should be intuitively obvious to the casual observer...",  I find that it is seldom all that obvious. :)

@ Karin:  Good point.  As with old houses, it is with baking, There is an endless number of unknown variables,  It becomes the craftsman to use his skills to get the hem right. The tape measure will assure you're in the ballpark with the amount to buy; enough material to reach the floor, and not so much as to be wasteful.  In this case, using a little math gives you a reasonable starting point.

cheers,

gary