Natural odds, by definition, would be the dart-throwing chances.
Thus...

• 8-horse field, natural odds would be 7/1.
• 10-horse field, natural odds would be 9/1.
• 6-horse field, natural odds would be 5/1.
• 12-horse field, natural odds would be 11/1.

However, I suggest that you use a minimum of 8/1 and a maximum of 10/1.

Therefore...

• 8-horse field, natural odds would be 8/1.
• 10-horse field, natural odds would still be 9/1.
• 6-horse field, natural odds would be 8/1.
• 12-horse field, natural odds would be 10/1.

Thanks that cleared it up.!
Btw, the P&P 2022 IVR, BVR and ROR have helped a great deal in me formulating my own aML and AMLQ objects using BRIS factors, for which I have found some of your own that are close enough proxies for them. Also been examining possible standalone or combined BRIS factor (Reynolds) rankings that are indicative of vulnerable favorites.
Been fun developing a spreadsheet application that automates much of the work and identifies contenders, playable races and final wagers.
Life begins anew at 72...LOL.

Life begins anew at 72...LOL. — Mickey Arnold

That's 2 weeks away for me!

Life begins anew at 72...LOL. — Mickey Arnold

I guess I am the youngun. I turned 70 this week.