What's that skill called? [later additional suggestion] if the 17th century "Computer" or 14th century "Calculator" names for master arith/math-matician is not what your looking for then we need to accumulate all your requirements to perhaps find a Greek variant that was commonly used prior to that.
logistician (modern uses, route planner / assigner of labour and resources)
Greek logistikos, skilled in calculating, from logistēs, calculator a variant of logician See current military usage
The first administrative use of the word was in Roman and Byzantine times when there was a military administrative official with the title Logista. At that time, the word apparently implied a skill involved in numerical computations.
Since others have mentioned arithmetic + mathematics and geometry seems not applicable The other applied forms I can think of are
Actuaries may solve your problem, Numerologists and Stasticians
a(re)bound by numbers. Strategists and Tacticians logically come close to your target and
just for good measure we have Tallymen
As answered above a computer  (one who computes) or computor was an early 17th century term. Alternatives include computist possibly used before 1646  or listed at a later date computant. Notable pioneers in this field included
Mary Edwards (c. 1750 – September 1815) was a human computer for the British Nautical Almanac Her daughter, Eliza Edwards (1779-1846), also worked as a computer [until...] Civil Service rules made the employment of women very difficult.
The supposed first digital calculator was "The Arithmometer" or Arithmomètre not introduced till Patented in France by Thomas de Colmar in 1820 long after computers.
So for you quiz masters "Which came first the computer or the digital calculator?"
My Great Aunts reputedly lived over the age of 100 and worked with the navy, in The Great War (WWI), but would never disclose what they did, however the family mumbled they were computists. It would have been fun to say they were ComputeAunts!
 see alephzero's comment below for citations e.g used by Sir Thomas Browne in PSEUDODOXIA EPIDEMICA, Volume II Third Book, Chapter XIV, Gutenberg EBook "we have the account of a year in 365 days, exact enquirers and Computists will tell us, that we escape 6 hours, that is a quarter of a day." "Now it is manifest, and most men likewise know, that the Calenders of these computers, and the accounts of these days are verry different;"
The Oxford calculators including Richard Swineshead "The Calculator" was the name given to a group of lead logical mathematicians founded Early 1300's