I’m trying to get my head around a correct formula for calculating ROI (annual).

As an example I’ll use Arctic coin (ARC).

At the time of my writing, these are the values for ARC (source = masternode.online):

- # of master nodes = 1,326
- difficulty 6974.541545346924
- ROI = 10.91 (this is the value that I should get from the formula)

From https://arcticcoin.org/en-EN/mining-pos , this is the formula for calculating ROI :

Per day = (n/t) * r * b * a
ROI (return on investment, in year) = ((n/t) * r * b * a * 365)/1000, where 


n - the number of personal active Goldmine (masternode), we’ll use 1 master node for this example

t - the total number of Goldmine (# of masternodes)

r - the current block reward 

b - blocks in an average day 

a - the average Goldmine payment

a is 45% (from the coin specs)
b is 576 (from the coin specs block generation = 2.5 minutes (150 seconds) so 86400 (seconds in a day) / 150 = 576)
r is apparently 2222222/(((Difficulty+2600)/9)^2), so plugging difficulty into the formula 2222222/(((6974.541545346924+2600)/9)^2) = 2094.459943449576

So the final formula ((1/1326) * 576 * 0.45 * 2094.459943449576 * 365)/1000 = 149.43639994711725

Waaaaaaaaaay out of 10.91 from above. What am I doing wrong ?

My other question is - is the above formula universal for all masternode coins ? I know that the block generation time and masternode reward percentage varies across coins, but could it be generally used for all coins ?

Thanks.