Because it works with the recommended dosage levels I don't think we need to change the dosage at this time.
I found an original research results of European Union Varroa Group
Oxalic acid treatment by trickling against Varroadestructor: recommendationsfor use in central Europe
andunder temperate climateconditions J.D. Charrière, A. Imdorf (2002)
Last modification: 10.01.2007
The Swish use 2,8% OA instead of many other countries 3,5%. Italy uses 4,2%
The volume is the same. Its duty is to make bees dirty and keep the acid contacted to mites enough long time.
The original recipe is http://www.agroscope.admin.ch/imkerei/00316/00329/02081/index.html?lang=en
30 ml to small hives
40 ml to normal hives
50 ml to large hives ( = 2 box)
Maximum 4 ml per frame gap and to 2 box hives 5 ml
I have talked personally with our Seppo Korpela what this means and he sais that Switzerland has lower percentage but in tests it gives almost same results. He was in EU varroa group as a researcher.
In Finland measures has bees the same 10 years. But something is happening now. Varroa has become more dangerous than 10 years ago.
It is said that assistant viruses are more lethal than 10 years ago. Lots of hives have died for varroa during last 3 years in very experienced beekeepers' yards. I have treated my hives 25 years. First year it was 1987 a swarm. Stuff was Perizin and it is used still whne researcher want to know how many living mites the hive still have.
Mite drop after treatment is difficult because dead mites are in empty comb cells after 5 months.
Oxalic acid alone is not enough to save hives. Late summer treatment with thymol or with formic acid is more inportant to save winter bees.
When these treatments are not successful, OA gives the final hit on mites and advantage is to bee seen next autumn.
But you see the results, when you give OA but you cannot see, how many living mites you have over winter. You may get 10 mites, 500 mites or 1000 mites. As guys say, varroa drop before treatment does not tell what will happen.
Canada is very experienced in Formic acid treatments but they have met heavy losses with mites. Reason is that treatment is not allways as successful as promised 96%. It is sometimes 80% or 70% and it is too much. Mite doubles itself in a month.