I'm sure I'm not the best one qualified to answer that one - I hope the real pros are in there somewhere
but happy to give it a go.
Most oil boilers are capable of delivering a heat output within parameters specified by the manufacturer, and there own specifications are generally available.
Here's an example of a Worcester Heatslave 20/25 which is a medium to largeish domestic combi.
Depending on how its set up, the output will be between 20 and 25 kW, obviously.
For 20kW output, Nozzle is .60/80 deg EH, pump pressure 130 psi
For 22.5kW output, Nozzle is .75/80 deg EH, pump pressure 105 psi
For 25kW output, Nozzle is .75/80 deg EH, pump pressure 130 psi.
(Notice in the last two, the nozzle is the same but the pressure differs).
One of the most important things is that the CO2% is between 11 and 12% which is reasonably standard, but as a rule on smaller outputs the CO2 will be a little less, say 10.5%.
The spray angle shouldn't be tampered with. EH means an 'extra hollow' nozzle (spray pattern).
Generalising, excess pump pressure causes overfiring and burning of the baffles, maybe with local boiling in the heat exchanger, too little pressure causes soot formation and excess CO2.
Let the others jump in now, but which boiler do you have and for interest, what are you trying to achieve?
John