substituting rosehip solution for lemon juice/citric acid
This is going to be difficult to boil down (excuse the pun) to an objective guideline, even a rule-of-thumb. The problem is the wide variation in the acid content of rosehips. 

This academic paper (which looked at varieties grown in Poland, but is fairly representative of what I can find; for similar examples see abstracts available here, here, and here) found a wide range in the two major acidic compounds which contribute to the tartness of rosehips:


Citric acid: mean of 3.16 with a standard deviation of 1.12, minimum of 0.20 and maximum of 5.37
Ascorbic acid/Vitamin C: mean of 1.06 with a standard deviation of 0.58, minimum of 0.08 (!) and maximum of 2.67


(All values given here are in grams per 100 grams of dried rosehip matter)

This is a pretty wide variation, and it means that it's going to be difficult to predict exactly how much of either compound the particular rosehips you have contain. Rosehips at the higher end of acidity could easily contain twice or three times as much acid as rosehips at the lower end, meaning the amounts you extract (even given a precise temperature and time of extraction) could vary widely.

My recommendation is to simply adjust this subjectively; you can either make more rosehip extract than you need and modify the amount to add the desired amount of tartness, or vary your extraction process to concentrate the extract. I would stick with the first as it will be easier to adjust on the fly.

Keep in mind that you will need to repeat this whole practice if you're making more syrup in future, unless you're using the same source batch of rosehips each time.