Sounds like your DD will adjust to formula no problem! It takes a couple of weeks for their tummies to adjust to formula, and that they will go for longer stretches between feeds. They won't instantly go to 3-6 hour stretches (unfortunately!!!).
Time to transition: I did it in one week from exclusive pumping to dry. When I got advice from MNers, some went cold turkey! Whereas Kellymom etc advise just switching one feed for formula, wait a couple of days etc. Thats way too slow and painful IMO. Straight away I switched to alternating a FF for a (expressed) BF. As soon as you can, drop any feeds that happen between midnight and 3am, when milk production is highest. When I did that I went in 3 days from getting 100ml off each boob in 20 mins down to less than 30ml! So in just a few days DS was almost fully on formula and we were just giving him the rest of the expressed milk we had in the fridge once or twice a day. Getting rid of those night breastfeeds is key.
Also watch yourself like a hawk for blocked ducts and mastitis!! Do you have a breast pump? When you feel you're engorged you could pump for 5 mons to relieve pressure and then give the milk to baby or chuck out. Get some cold relief pads for your boobs too, this will really help. Hand express in the shower to relieve discomfort.
As for amount: go with the guide on the formula tin for age/weight of your baby to start with, this is what I was told by HV. For the first few weeks he was taking way more than that - feeding every 2 hours but it soon settled down.
Surprisingly the best and quickest regular advice I got on feeding queries and transition to formula was from the free Aptamil chat helpline, rather than from any health officials... Go figure! I'd really recommend them, actually. They (and a paediatric nurse friend of mine) said that its not likely with a modern whey dominant formula that the baby will get constipated BUT if baby does not poo for longer than 3 days or has hard poos then offer 20-30ml cool boiled water between feeds.
My DD is 4 months, was ebf from birth and in anticipation of going back to work when she is 6 months, I have decided to introduce formula. I want to make the transition slow as I have had problems with engorgement and oversupply, but its difficult to know how to do it. Luckily she takes a bottle no problem (in fact it is so much easier than breastfeeding her when she fusses and his constantly distracted but that's a different story!) and so I've tried her with a bottle of formula for the last feed of the night since last Friday. She has taken 3-4oz and then would not have any more and she was sleepy. But then she woke up an hour later and seemed hungry so I gave the breast. Probably this was the wrong thing to do when I'm trying to reduce my supply, but well, that's what I did. Last night she took 4oz, slept at 7pm, woke up an hour later and I gave her more formula (I think she took about 2oz) but then had a disrupted night up at midnight, 2am and 4am when each time I breastfed her. I know she can go through the night with just one feed and had slept for 7, 8 even 10 hours on occasion. So really I'm looking for advice about how to stop breastfeeding gradually and how much formula she should be taking. Although she's 4 months she's only little weighing about 12lb. My next step was going to be replace the each daytime feed one by one over several days.