IH8TOADs, turbotrana, hiddeous, thinking about it I am doing what Toads says at the end so am just fudging as you say.
The way I get the injector dialled in was to find an injector that was close (I had Bosch 968s and used the data from the Ford M39 injector with the maximum value for Offset from the BA and M39 calibration). Increase Offset until the idle smoothed out. I would then adjust the Low Slope and breakpoint until I go the idle fuel trim under 2% (bit of a tedious process, but calculated by maths).
To Dial in the tune:
Drop boost to 10psi and lambda back enough so that the injectors were below 90% duty cycle (constant 0.75 from 2500rpm up was OK for them) and do a full throttle run to get the High Slope dialled in close. Another full throttle run and I would do a Lambda difference on the desired versus actual and multiply the "MAP per Airmass" table in those RPM ranges by the ratio of desired/actual to clean up the range (outlined in the Speed Density write up I did).
The car would be running well by this stage. Because I don't have access to a dyno then I would drive (200+kms) and get the LTFT values around all the breakpoints that covered the cruise range (1250-2000 covers the 60-110kph range on my car, so breakpoints of 1250, 1500, 1750 and 2000rpm). I log the LTFT/Cam Angle/RPM was for each point and adjust the "MAP per Airmass" at each of these points ("MAP per Airmass"/(1.0+LTFT) so if fuel trim is -7% I would divide by 0.93).
This last step was the key for me getting the fuel economy as I would get my cruise fuel trims under 2%. If I had a dyno I would have done what Tony/LaSota racing suggested and set the LTFT off and measure the STFT to get the amount out at cruise. Note you can do a quick and dirty here and just work out the fuel trim adjustment that will get all the load points close to 0% if you wish.
So in all a fudge that will get you really close.