How soon would you treat new bees going from Nucleus into a Flow Hive 2, let them settle for a couple of weeks and then do the first treatment? this is a new hive though will not have the bees until May 2019
I have been looking at this treatment
https://www.bbwear.co.uk/bee-supplies/treatments/varroa
I would leave them at least that long, and maybe a month. Rather than treating empirically I would do a mite count first using either a sugar roll or alcohol wash, and treat according to the counts.
I don’t really like formic acid treatments, especially for new hives. They can be quite hard on the bees, and most experts say that there is a 10% risk that you may lose your queen. I far prefer oxalic acid vaporization. Once you have invested in the equipment (quite expensive), each treatment only costs a few pennies. It is quick to do, and minimally disturbing to the bees. Treatments can be repeated at 5 day intervals until you have good mite counts. 
Thanks Dawn I will have a look
I have the Swiss-designed Varrox unit, and it is excellent. Expensive, but durable, reliable and nicely made. You can buy one here:
https://www.thorne.co.uk/index.php?route=product/search&filter_name=varrox
You then need a power supply, and I use a lightweight jump starter, as one of my hives is a long way from civilization. Any 12V jump starter designed for SUVs, trucks etc, should be more than good enough. Mine will do about 5 hives before needing a recharge.
For the Oxalic Acid, I just use commercial wood bleach. In the US, this is 99% pure oxalic acid crystals, which is more than good enough. In the UK, B&Q or Homebase should have something equivalent. 
Thanks again Dawn I will have a look
What is the jump starter you use? Having trouble finding a reliable one!
I use this one. You have to keep pressing the Boost button every 30 seconds, but it is simple, reliable and light weight.
Thank you! This one isn’t available but I can find one similar I’m sure. Thanks!
A small tractor battery works too 
If anyone else should be looking for a jump starter to power their oxalic acid vaporizer, I ended up going with this one from harbor freight. You do have to push the jumpstart button every 60 seconds but it will definitely last through several treatments on a single charge
Update: the viking jump starter lasts for 5, 4 minute treatments, or a total of 20 minutes of power on a single charge. And if anyone is looking for an oxalic acid vaporizer that fits the opening of the flow hive 2 this one is it! And less than $30!
BBTshop Oxalic Acid Vaporizer… https://www.amazon.com/dp/B07S5CC4CM?ref=ppx_pop_mob_ap_share