It's hard to say because hpts are not a quantitative test. They simple show whether or not there is any hCG in your urine and the box even says not to use them for tracking progression because there are so many variables that can cause the hCG level in your urine to vary.
I understand you are worried about another loss and it's nice to see that progression for peace of mind. BUT trust me when I say this-pee sticks don't tell you diddly squat at this stage. (I know this from a decade of personal experience in the infertility and recurrent pregnancy world.)
Your best bet is to step away from the hpts. Hard I know but it's possible. If it eases your mind, go for regular betas instead. You'll get a definitive number instead of a line that can be influenced by what number wee it was, how long of a hold there was, how much fluid you've had, how old the test was or how strong the dye is or how strong the antibody strip is...I've been horribly burned by hpts in the past so I do not use them for progression. It's not worth the fear.
And for the record, your numbers are fine and you are still doubling normally. I'm not sure when you had your first beta done but here's a quick breakdown that may help a bit.
For example, if you had your first beta done at 14dpo and your doubling time is 48 hours, you'd see the following...
14dpo-40
16dpo-80
18dpo-160
20dpo-320
You are well over that 320. Even if you had your first beta done at 12 or 13dpo, you are still doubling just fine because a 'normal' doubling rate is actually just a rise of at least 60% over 48 hours. And not everyone doubles according to that. I know I don't. I always stay on the lowest end of the 'normal' hCG ranges and if I continue doubling every 48 hours beyond 18dpo, then I'm doing the best I've ever done.
I know all too well what a history of miscarriage can do to you though and I know how much those tests seem like a good idea. But it's honestly not worth the stress and the fear and the anxiety so stick with betas and once you hit the 6.5-7 week mark, go for an ultrasound to see the hb.