as you already indicated multi-tabling is generally going to lower your ROI but increase your hourly.....as for strategy it shouldn't effect it although the more tables you play generally you will be playing a tighter range since you will not be able to monitor every move at the table or take a lot of time to make some tricky moves you could if focused on only a single game.....granted this is talking mass multitabling like 12 tables or so as once you get used to multitabling even 6 tables could sometimes seem slow when talking 9max although 6 tables with 6 max will often keep you on your toes enough not to fall asleep
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The formula is not really a formula but more some simple logic if you think about it.....as a basic example if you at playing 10$ games with an ROI of 10% while two tabling and it takes 1 hour for each game then your expected return for 1 hour would be 2$....Buyin * ROI * # of tables....10*0.1*2 = 2$.
If you increased to 4 tables and say your ROI drops to 8% then your expected return for the hour would be 10 * 0.8 * 4 = 3.2$ so although your ROI dropped you actually increased your hourly by $1.20....a 60% increase.
Everyone has a sweetspot when it comes to the # of tables they can play which provides the the best hourly for them, for some this could be 6 tables or 10 tables or maybe only 2 tables but with practice and experience it can certainly be found and possibly increased with work on your game.
The sweetspot for multitabling is the point where the # of tables you play provide the best hourly return at the limit.....as mentioned increasing tables will lower the ROI but increase the hourly however this is not always true because beyond the sweetspot your ROI may drop to a point where your hourly is now lower than it could be playing less tables.......
For example in the image below....lets say for every 2 tables you add your ROI will drop .5% and you started with a 10% ROI grinding 2 tables...essentially the data in yellow would be the sweetspot for this example grinder. As you can see 20/22 tables would be a sweetspot as more tables actually lowers the hourly, granted some of this could be offset if there is rakeback or vip level bonus' etc to consider but based just on ROI/Hourly 20 tables with a 5.5% ROI would be the best hourly the player gets at the 10$ level.
**Just know to calculate this for your own grind you would need to put in lots of games, 1k min at the current level and # of tables to get a decent idea of your ROI and then increasing tables for set sample's until you could essentially plot this example graph on your own data to find your sweetspot.
Hope this helps.