If you think having a round of about 100 jobs averaging £35, servicing about 5-7 customers a day or 25 a week is possible, your half way there already. It’s just a matter of time
(that will differ slightly with different people)
If you think you it’s not possible or you can’t get your head around it, don’t waste your time trying, (until you can) 
There is absolutely no need for an independent window cleaner (sole trader) to have more than 200 customers on his books; it just means he’s got his pricing and how he operates his round wrong. (Fact so bother arguing) Because your to busy cleaning hundreds & hundreds of customers relentlessly. 
You have a choice, keep cleaning windows day in, day out etc (because you do love your customers) or do that and build “your ideal round” for you (your family, your future, your retirement, your home, and any other reasons) That’s where the real work needs to be done, not getting yet another (pointless) customer (yawn). That increases your work load, but does very little to the profit margin, absolutely the wrong reasons to even think about taking someone on. (You’re not even there yet as a sole trader) 
IMO 
I regard my round as fairly full with about 245 customers and some subbying on top. You're right though - I could tailor it more towards larger properties and reduce the customer numbers. As things stand, I would think 300 plus the subbying or maybe 340 - 350 without it would be very full.
However, larger properties can have their own drawbacks - mainly concerning access. I try to not get too involved in work that must be pre-arranged. I have eased off on that a bit in more recent times but having to pre-arrange work means taking an educated guess at how much work will be carried out in a day. If I overestimate it means letting someone down. If I underestimate I haven't used the day to its full potential. Therefore, I feel that the best way around this (for me) is to either do very little work like this OR to underestimate and to have some smaller filler jobs (no pre-arranging required) that can be tacked onto the end of a day.
So far, I have chosen to have very few jobs that need pre-arranging.