What I usually do is decide my entry price, in "normal " environment i use 1-2% price dips to purchase additional stock.
So lets say my entry price of apple is 120$, as we have volitile market i use 5% mark.
So I will set limit buy to 120$ 1 stock.
Then I calculate 5% discount.
120x5/100=6
120-6= 114.
I set next limit 114$,
Then 5% discount on 114$.
114×5/100=5.7
108.3$
I set limit for 108.3$.
Same process for other stocks. 5% discounts are used only due to fact that we are having 5-10% daily swings.
In normal ocasions 1-2% would also be fine 
Agree fractional limit buy would be awsome, I belive this should be part of major update with automation of porfolio.