## mathematical calculation tracker field

Having difficulties with this one - one time worked, another no luck. Most likely my very limited knowledge of mathematical formulas . This could be a 'documentation' forum issue (for mathematically challenged people like me) as not that many examples in either this specific topic or Advanced Rating (language) page.

tiki 12.2

farming topic - trying to calculate difference (so 'subtract', but will use all eventually, i.e. -,+,/,*) for tractor engine working hours:

tracker Id24

field Id338 (numerical field), name: moto_start (sample number 500)

field Id339 (numerical field), name: moto_end (sample number 509)

field Id340 (math.calculation), name: total (total should = 9)

I got field names to work on another tracker once, but seems field id's would be more consistent, i.e.:

(sub f_339 f_338) or something like this.

I tried may other ways, most of the time gave me an error about not able to find the name of field, i.e. "moto", but also gives: Expecting "(" near ""

so also tried:

(sub (f_339)(f_338))

(sub (f_339 f_338))

(sub moto_end moto_start)

(sub (moto_end moto_start)) (sub (moto_end)(moto_start))

thanks,

Mike