Constipation since moving to formula

Constipation is not unusual with formula but can be uncomfortable. It is good that she is passing 1 stool each day. What is the consistency like. Hard, small pellets, soft like toothpaste, watery etc?
Unfortunately there are little evidence based options here. Anecdotally I would consider the following -
As a first option I would try some gentle abdominal massage to her pass a stool. You can perhaps include this as a pre or post bath activity.
You also might find more tummy time may assist her.
You could try switching to a ready made version of the formula and obviously check that you are making up the formula correctly.
Is she struggling to pass the stool or is it largely discomoft associated with not having opened her bowels? If she is struggling to pass the stool then you could try holding her on a potty (with nappy off) as this may be an easier position for her to pass a stool in.

Thanks Lisa! Consistency is between pellet like and toothpaste I would say - today she managed to go three times so it’s getting better but she is still very very gassy and gets very uncomfortable. Would you suggest increasing the probiotic dose to two sachets instead of one while this transition to formula is still early?

Again the evidence base doesn't exist for this but optibacs reccomended use says you can increase to 2 sachets if desired so it may be something you can experiment with. I would also try some massage and upright positions too.
I am assuming you are paced feeding too. Are you comfortable with that method of feeding?