I, too, have been unsuccessful in finding the scaling factor for safinamide.
NICE  report on safinamide states:
“A range of adjuvant treatments are available for people with Parkinson’s disease who are experiencing motor fluctuations on levodopa alone, including other MAO-B inhibitors (rasagiline and selegiline).”
" … safinamide has not been compared with other active treatments in a head-to-head trial, so no information is available on how its efficacy and safety compares with other treatments, particularly other MAO-B inhibitors. The EPAR states that indirect comparisons of data on safinamide with historical data on other treatments, such as rasagiline … were considered favourably".
“Safinamide differs from other MAO-B inhibitors in its mode of action, as it also has additional activity at non-MAO-B targets. However, the EPAR concluded that at therapeutic concentrations, inhibition of brain MAO-B was the most likely mechanism responsible for the observed increase in on time.”
Once you have disabled all the MAO-B, increasing the dose of an inhibitor has no further effect. For rasagiline this threshold is approximately 1mg, giving a benefit equivalent to a dose of 100mg levodopa.
But, and this is a very crude estimate, if pushed I would suggest that both a 50mg and 100mg dose of safinimide would have the same LED estimate as 1mg rasagiline. From this it follows that you map both 50mg and 100mg of safinamide to 100mg Sinemet (L/C).
So, to answer your question:
600mg Sinemet plus 50mg safinamide has an estimated LEDD of 700mg
The LED measure is based on the pharmacodynamics of the drug concerned (what the drug does to the body). If you are interested in the pharmacokinetics of safinamide (what the body does to the drug), you need to take into account both the absorption of safinamide and its effectiveness at inhibiting MAO-B. Again a rough approximation is to assume that because the half-life of safinamide (23 hours , compared with approximately 90 minutes for levodopa) is so long its efficacy is constant throughout the analysis.
My apps don’t include safinamide, because of this lack of data. To use my apps , the basic LED calculator, and , the graph drawing program of LED changes through the day, the work-around is to just enter 1mg rasagiline instead.
 NICE advice on safinamide, Feb 2017
 Levodopa Equivalent Dose Calculator
 Levodopa Equivalent Minute to Minute Plasma Graphing Tool