I have never worked as a setter Spice, but my Grandad was a bookies runner in the days before betting shops were legal. He used to have to set the odds as he was "doing his rounds" to reflect his bookie's liabilities. Without going into great detail about "Vig" and "Overround" it would be difficult to nail without the readers having pretty good maths skills. Needless to say the odds 15/1, 17/1, 19/1 etc make the quick maths required, to make the book % Overrun, right for the bookie get more complicated. I know they have computers now but the tradition has stuck in some ways. Incidentally I looked at the betting history of the "horse" mentioned right up to 1 minute after midnight on the morning it was due to run . The best odds ever offered were 57/5 on an obscure Asian site and it touched 11/1 briefly on Skybet and Ladbrokes (plus a few other very low volume sites).
PS My Grandad used to get me as an 8 year old to check his maths, before going over to the butcher's( bookie) to hand over the loot and pick up his "meat". The butcher used to give me a few pennies, for keeping tucks and adding to the everyday innocence of the operation.