The Final Rise

there are lots of variables:

1. the type of flour: 100% rye ferments faster than other grains- I get a fully activated starter 4-5 hours after feeding my base culture.  Others report 8- 12 hours if using white flour or conditions that are not otherwise optimal.  This is not to say that you can't get a 5 hour ferment with white flour, but all things being equal rye will ferment quicker due to beneficial enzymes that are not present in other flour types.
2. the build time matters: building stage one from 50 grams to 200 over five hours, then adding enough water and flour to build up to 500 grams total will give a better result than starting with 50gr and adding 450 of flour and water (at your target hydration percentage)
3. temperature - on the counter at 70° vs. say in the oven with the light on creating a nice 78° environment for those little happy bacteria cells
4. the percentage of pre-ferment in the recipe - 5% vs 20%, the latter clearly having more cells to start working on the rest of the mix after kneading, etc.

There is more others are sure to add, meanwhile it sound like you are having great results!!!

Try the extended build times.  I've stopped using commercial yeast and get better results, more flavor and more easily digested bread. 

Build stage one above for 5 hours (50gr to 250gr or so, or approx 15% of your recipe).  Then build stage two targeting another 20-25% of the recipe to a total of 45-50%.  Let that go overnight or at least 8 hours.  Add the remaining ingredients, mix until incorporated.  Let rest 30 minutes, then knead dough for 6 minuites.  Let rise covered in oven w light on for 2.5 hours with one gentle stretch and fold half way through.  Preshape loaves, cover with plastic wrap on board, let rest 20 minutes.  Shape final loaves by tucking in all edges to center so dough is deflated and stretched, turn over and round loaf and place in basket top side down (plastic colander or bowl works too) lined with a floured dinner napkin (a few dollars next to the tablecloths at you favorite store- synthetic material is best).  Place each basket in a large plastic bag and refrigerate for 8-15 hours.  Remove from fridge, preheat oven and stone for one hour at 470°.  By now the loaves are well risen.  Put a piece of parchment paper on top of the bowl/basket, then holding both, invert so dough is on the parchment, gently remove napkin.  Slash, and bake.  Steam oven at loading, and three times four minutes apart (I open the oven door quickly, and throw one third cup of water on the floor, do not hit lightbulb!), then quickly shut the door.  Bake 45-55 min depending on size of loaf.  I like sticking a probe themometer into the bread, until internal temp is 202°.  So 2 and 1/2 days total time to make this!!!  But you will get a nice very nice well developed and complex flavor, with more than typical sour- guaranteed to blow the doors off of a one day bake.  Shorten ferment times at all stages for less sour...

use the search box for 1 2 3.  essentially you divide the recipe into 6.  stage 1 is 1/6th or 16.7%, stage 2 adds 2/6th, or adds 33.4% to stage one resulting in 50% being used.  Final stage adds all of the rest... 

Use 100% hydration ratio in stage one (equal weights not volume of water and flour).  Stage 2 can be thicker making the entire batch of 50% like oatmeal, all of the rest going into stage 3.

Works very well!

Without getting too scientific about the whole thing:

Whole grain breads and transitional whole grain/white flour breads usually should rise 1 1/2 times their original size during the final rise.

White breads, depending on the recipe, should rise 1 1/2 times to twice their original size. They are sufficiently proofed when a dent made with your finger doesn't close, but comes back only very little, if at all.

Watching your dough is better than any anxious watching of the clock!

Karin