'quick and dirty' estimate (that is pretty darn close!) is to subtract 1/3 of your CYA if your pH is 7.4-8.0 and 1/4 of your CYA if your pH is 7.0-7.2 from you TA. This will get you within a few ppm of the adjusted TA done by the mathematical formula. With your CYA as high as it is I would not let the unadjusted ALK fall below 110 and would run it as high as 140 ppm at a pH of 7.4 or higher!