Hi there, Bill,

Just my two penneth – I'd say the 'spot' needs to be big enough to at least fully contain the chisel point on the first proper drill you intend to use.

Best5 regards,

Swarf, Mostly!

Really depends on how you want to treat the hole.

A spoting drill can also be used to camfer the edge of the hole prior to it being drilled thus saving one operation in which case an 8mm min or preferably a 10mm 90deg spoting drill would be the best option so you don't have to deburr the entry side..

Larger still if you wanted a fully countersunk hole as the spotting drill can also pre countersink rather than going back after drilling with a CSK bit.

I would not really class the holesaw pilot drills as spotting drills, maybe 4-facet or split point but a bit long for spotting so may wander a bit. I mostly use an 1/8" one which does for starting holes upto about 5mm, 5mm one above that or 6mm carbide one in the CNC.