I understand that the lower the tension the higher the power you obtain from the string bed. The question is how low can you go before you start losing power from the strings being too slack??
Obviously it would depend on the string thickness and type used but just wondering what the approximate value would be.
From my own experience, I would say around 20-22 lbs for an adult ... 18-20lbs for children .... bear in mind, you will lose around 2lbs in tension within a few weeks, so, give yourself some leeway of +2lbs when stringing.
Personally, my optimum is around 21x22 lbs using 0.66mm (BG66) or thinner (0.63 Gosen R4x100), so these repulsion strings do add a bit extra power as well.
The range is very wide, depends on the racket's weight, balance, stiffness, string's thickness and player's technique and physcial condition. There will be players consider 26lb is "way too loose", and there are others thinking 23lbs is "way too tight".
If someone comes to me, have no idea what s/he is asking for, I usually string (with BG65) 23 or 24lbs for adult male players, and 1 or 2 lbs lower for female / junior players. This is the under the assumption they are beginner / club level players. If they feel need some adjustment for the next job, I will be glad to adjust for them based on the feedback.