It looks to me that you're doing pretty darn well, and I think I'd keep on with what you've been doing.
I also found this from another cat veterinarian in answer to a similar weight question;
By 5 weeks, they should be just over 1 lb (which is 454 grams). I would expect them all to be between 450 and 500 grams at this age. Kittens gain about 1 lb per month. So, at 1 month they usually weigh 1lb, at 2 months, 2 lbs and so on.
I also found this from another cat veterinarian in answer to a similar weight question;
By 5 weeks, they should be just over 1 lb (which is 454 grams). I would expect them all to be between 450 and 500 grams at this age. Kittens gain about 1 lb per month. So, at 1 month they usually weigh 1lb, at 2 months, 2 lbs and so on.