How to calculate doubling time?

I know HCG should double every 48-72 hours but I do not understand the percent increase that I see others talking about, can someone explain? My first beta at 10dp5dt was 78 and at 14dp5dt it was 330. They told me levels rose appropriately. I am 5 weeks today and won’t have another beta until 6 weeks and also first ultrasound. Hoping for a sticky baby. They seem to be similar to my previous FET that was successful so I’m hoping they are okay. My previous cycle numbers were 91, 229, and then in the 500’s, had a healthy pregnancy. First ultrasound was not until 7 1/2 weeks though so I’m a little nervous going earlier this time and if we can see anything. I was more oblivious first round and didn’t worry too much about the numbers but this time I’m more nervous and just hoping.