Max gross weight

[poadeleted20]

So if I use approach flaps (20 degree) that would make a significant difference ?

The use of less flap than the book recommends for normal landings will usually make a significant difference in the climb gradient in the landing configuration. Here's the actual rule on that for the planes we're talking about:

Sec. 23.77

Balked landing.

(a) Each normal, utility, and acrobatic category reciprocating engine-powered airplane of 6,000 pounds or less maximum weight must be able to maintain a steady gradient of climb at sea level of at least 3.3 percent with--
(1) Takeoff power on each engine;
(2) The landing gear extended;
(3) The wing flaps in the landing position, except that if the flaps may safely be retracted in two seconds or less without loss of altitude and without sudden changes of angle of attack, they may be retracted; and
(4) A climb speed equal to VREF, as defined in Sec. 23.73(a).

Note that C-172's with the 160HP engine and 40 flaps have a MGW of 2300 which can be raised to 2400 lb by installing an STC'd modification which limits the flaps so they'll only go to 30 degrees. The only thing that changes is the flap selector/limiter -- no structural changes and no other limitation changes. This suggests that the limiting factor on those 2300 lb 172's was the climb gradient requirement, and gives you an idea of how much difference that last 10 degrees of flap makes in its performance.

BTW, for those wondering about the CAR 3 standard, here it is for comparison with the current Part 23 standard:

(c) Balked landing conditions. The steady angle of climb at sea level shall be at least 1:30 with:

(1) Take-off power on all engines,

(2) Landing gear extended,

(3) Wing flaps in landing position. If rapid retraction is possible with safety without loss of altitude and without requiring sudden changes of angle of attack or exceptional skill on the part of the pilot, wing flaps may be retracted.

Note that 1:30 is the same as 3.3%, so the standard is essentially the same.