Optimality factor

Definition 1. The optimality factor (op) of an opening / position is just the number of optimal moves.

The optimality factor of a k-acute opening

https://www.chess.com/forum/view/chess-openings/acuteness-of-openings

is equal to k. A move will be considered optimal if it within 0.1-0.2 of the best evaluation. Well, it's somewhat ambiguous depending on engines, depth etc. I'll use Stockfish 8 here (d=20, 9 lines) and elsewhere (d=20/20, 5 lines).

The optimality factor of the initial position is equal to 3+ (optimal moves are, at least, 1. d4, 1. e4 and 1. Nf3). Here are optimality factors of most popular openings in 2 plies:

• 2D 1. d4 Nf6 Indian, 416K+, op=3+ (2. c4, 2. Nf3, 2. Bf4)
  
• 2Ec 1. e4 c5 Sicilian, 395K+, op=2+ (2. Nf3, 2. Nc3)
• 2Ee 1. e4 e5 KPG, 190K, op=2 (2. Nf3 and 2. Nc3)
• 2Dd 1. d4 d5 QPG, 170K+, op=3+ (2. c4, 2. Nf3, 2. Bf4)
  
• 2E1e 1. e4 e6 French, 114K+, op=3+ (2. d4, 2. Nf3, 2. Nc3)
  

(to be continued)

 All these op-values are debatable.

What makes a move suboptimal ?

The difference between the evaluation of a suboptimal move and the evaluation of the best move should be higher than 0.2.

Thanks.