[LexxM3]

No experience with Dinan spacer and as stated, they don't seem to be known for spacers. Just for the record, I am using forged BLOXSport hubcentric spacers for winter aftermarket rims in a square setup that matches final stock offsets and am both impressed and happy with them after two seasons.

As for bolt lengths, stock bolts are 27mm and if you are using stock rims, spacers just increase bolt length required by spacer thickness. Approximately. So 12mm spacer ideal bolt is 39mm and 15mm spacer ideal bolt is 42mm; both are sufficiently close enough to 40mm that it makes 40mm a good simple, consistent, and available choice.

Note that if rims are not stock, all bets are off as their bolt hole may well not be at all the same thickness as stock and may alter required bolt lengths — that was the case in my winter rims situation, for example.