I total up my full bosul needs, ie for food, corrections + any other variables using my normal ratios. Then I apply an overall factor in anticipation of my activity I hope to achieve, perhaps reduce that total dose to 40%, more normally 50 or 60%, rarely dose at 100% (I'm rarely inactive); this activity factor is subjective of course but I'm finding I am getting quite good with the 'guess'.
Whatever my final no of units after the activity factor, say 7.5, I would now increase by 20%, which would make 9 units in this case. I'll round up or down a part number, say 9.2 for my half unit pens, depending on how much time I can give to monitoring; if sitting around I'll strengthen up and if gardening or walking to the post box, I'll probably weaken slightly. This is partly because the activeness makes it a little more difficult to keep checking and partly because any activity improves my insulin sensitivity (or if you prefer reduces my insulin resistance - one is the reverse of the other) and I'm slightly more likely to go hypo.