Best range for "all" natural guts is generally going to be in the range of 40-55lbs. Natural gut holds its tension quite well. The thinner gauges will hold tension even better and should be strung slightly looser. Although some players enjoy higher tensions which is fine and nothing wrong with it, but not everyone can adjust to the higher tensions and loss of power. This is the reason for the recommendation of lower tension and increase in durability with all natural guts.
15G V5 35-64Lbs (Ideal Range...45-58Lbs)
16G V5 35-62Lbs (Ideal Range...40-55Lbs)
17G V5 35-58Lbs (Ideal Range...35-53Lbs)
18G V5 30-48Lbs MAX (Primarily best for Squash and Badminton Players or light hitting low tension tennis players) No Guarantees on durability or reliability with this gauge due to its thinness)
16G ATP V5(+) 35-70Lbs (Ideal Range Open, Or 40-60Lbs)
17G ATP V5(+) 35-65Lbs (Ideal Range Open, Or 40-55Lbs)
16G ATP V5(+) and 17G ATP V5(+) Poly Hybrids 35-65Lbs (Ideal Range Open)
All High Performance NGW Monofilament Polys 35-70Lbs (Ideally Open to player style)
Some things to add regarding NGW High Performing Monofilament Polys:
1) They are the most Durable
2) A much softer, easier string on the arms and play phenomenal when combined with Natural Gut
3) Increased Spin and Control, but less powerful. Another reason to Hybrid with Natural Gut (Natural Gut suggested on the mains) will increase the loss of power with Polys. Generally the thinner Poly the better and best placed on the crosses.