1.3 times the stall speed is used on a lot of larger aircraft. There are two things to keep in mind:
1. The margin of 30% between the stall speed and the approach speed represents quite a few knots if the aircraft has a high stall speed, as with airliners. But, it is only a few knots on slower aircraft. A 30% margin might not be enough on some aircraft.
2. That 30% margin is intended to be a margin of 30% calibrated airspeed, not indicated stall speed. Many aircraft have airspeed systems with very large errors at slow speed, so if you simply multiply the IAS at the stall by 1.3 you might have an approach speed that is quite slow. For example, I sometimes fly a C182Q. The POH shows a stall speed of 45 kt indicated airspeed, or 54 kt calibrated stalls speed, with landing flap, at max weight and forward CG - in other words the ASI is reading 9 kt too slow at the stall, and that assumes no instrument error. In reality, the one I fly indicates about 40 kt at the stall under these conditions, so I think there is some instrument error too.
If I simply multiply 45 kt IAS times 1.3, I get 58.5 kt. The airspeed calibration chart says that 58.5 IAS would be about 61 kt CAS. If the stall is 54 kt CAS, 61 kt CAS is only about 13% above the stall. I've done approaches at 60 kt IAS on this aircraft, and there isn't any margin for a gust, or a pilot error. This is too slow for a normal approach speed, on this aircraft. 1.3 times the actual indicated stall speed of 40 kt wouldn't leave enough airspeed to flare, in my opinion.
Bottom line - don't put too much stock in the 1.3 times stall speed rule of thumb, unless your flight testing shows that it works for your aircraft.