The 555 varies pulse width approximately with the signal from the MAP. The MAP pressure is approximately equal to the inducted air weight. You should get approximately the right amount of fuel with this scheme at any throttle opening. It is non linear and imperfect in many areas, but with a WBO meter and a leaning knob, should be OK for our use. The circuit could be changed to make it more accurate, but then it will get increasingly harder to set up. At a guess, a straight 555 and MAP sensor might be accurate to +/-10% across the full power range. It will likely be leanest around peak torque, which you need to watch. If you really want to make it more accuratre, it's probably easiest to build the basic circuit first, test it and work out what correction syou need where, then ask a friendly geek to design a correction circuit. But it still won't be dead nuts.