I have no idea how tolerant the system will be. What you could do is use an adjustable power resistor and tweak it. There are some large ugly brown power resistors with a lug that you move by loosening a clamp with a screwdriver and moving the lug. That would allow you to adjust it to 7.5 Ohms or whatever it was I calculated, then if that doesn't work with all lamps burning, do a little trial and error.
Much engineering is done through trial and error. The first real job I ever had in 1974 was as an Associate Electrical Engineer. I worked with a very experienced engineer on an extensive high speed (for then) digital logic card for a nuclear medical system. Once we had the logic right he said: "well I think we've got it, all we have to do is start hanging capacitors on it, until it works." The application of decoupling capacitors was difficult to engineer and had to be done by trial and error. Science only goes so far sometimes.