Erm I find it Easier if one uses %'s rather than ratios. 5% was proven good for early 70's TZ's. Remember: it's Only too much oil.. when the plugs foul.
Isn't it a fine balance between robbing the engine of power (too much oil) and seizing it (too little)? The most power is to be gained by actually using less oil (so I've read), but obviously there is a huge risk involved for road use unless you have very deep pockets and bags of free time!
I thought it would be 5000 divided by 41(the 40 parts petrol plus the 1 part oil)
The larger number refers to the amount or "parts" of petrol.
The smaller number (usually 1) is the amount or "parts" of oil you will mix into it.
40 : 1 is the same as saying 40L : 1L
If you were to buy 40 litres of petrol you would need to add 1 litre of oil to it for a 40:1 ratio
To help you with the mental arithmetic...
You can divide each side by half until you get sensible numbers (so the amount you buy will actually fit into you fuel tank / can)
40:1 = 20:0.5 = 10:0.25 = 5:0.125 = 2.5:0.0625(L) - 0.0625 of a Litre is 625ml (just times by 1000)
Or another way is to first convert all the numbers from Litres to Mililitres by adding THREE zeros to each side like this...
40000:1000 = 20000:500 = 10000:250 = 5000:125 = 2500:62.5(ml) so 10000ml (10L) of fuel needs 125ml oil
and if you knock a zero off, for much smaller amounts you might want to mix...
4000:100 = 2000:50 = 1000:25 = 500:12.5 = 250:6.25(ml) so 250ml fuel needs just 6.25ml (six and a quarter mililitres)
basically its just a case of picking the units you are using first, then divide each side by have till you get to the amount of fuel / oil you are actually using - no need for a calculator, but if you want to check your answer if you get confused by all the "0." stuff, just times by 1000 and it should look right e.g. 0.125x1000=125(ml)