NoTan and Bad Proportions

While that sounds like the title for a Jim Gaffigan comedy special, the subject here is getting the most from those all important thumbnail sketches that usually make or break a painting.
 First, is a Notan the same as a thumbnail? Not really. Notan is strictly B&W and shows you if you have  a big, eye catching silhouette formed by your darks and lights (or not). They're often beautiful as is. Perfect if you're outside facing a backlit subject. Less perfect with strong cross lighting and cast shadows. But almost useless if it's overcast, or the sun is behind you early or late in the day.
 Desperate situations like that are a job for Captain Thumbnail and his Valiant Values. Thumbnails add the very necessary info about all those middle values - and this is important - whether they have an affinity for the dark side or the light side. Once a few of them sneak into a value area where they don't belong, mayhem ensues.
 Just remember the lightest part of a dark area can't be lighter than the darkest part of a light area. (I know I'll get challenged on that. We can settle it with colored frisbees at dawn...but that's a post of its own).
 Bad proportions...Confucius might say "Notan no good if proportion off".  Jessica Kirby puts it better than Confucius in her post here http://jakpaints.blogspot.com/
 It got me to thinking of easy ways to ensure a Notan or thumbnail matches the dimensions of whatever you're painting on. Obviously, a sketchbook of the appropriate proportions lets you  dive in without a thought, and I'm making my next sketchbooks that way. Fear not. You won't need a small library of sketchbooks. Just 2 at most...actually one with simple modifications. 
 Most common canvas sizes are just one of two basic ratios: 3:4 or 4:5.
The 4:5 ratio corresponds to 8x10, 16x20 and also 11x14 if you crop a WEE bit off the long dimension (or even ignore it, the difference is so negligible a decimal).
The 3:4 ratio corresponds to 9x12, 12x16 and 18x24 (again, if you crop a wee bit off the short dimension It will cover 5x7 but with most sketchbooks your Notan or thumbnail will be bigger than your work!)
 BUT...what if you don't WANT to make your own sketchbooks, or just want to carry one sketchbook? Well, there's probably already an app that does all this on a phone for the laziest of you out there but don't expect me to look it up and insert a link.
 For the more ambitious, do this:
1. Grab your existing sketchbook and measure the SHORT DIMENSION (this may go easier if you use a metric ruler - most rulers have both nowadays).
2. Multiply that by 1.33 (convert to inches if your not using the metric ruler). Find that point along the LONG DIMENSION of the page and place a piece of tape at that point on the cover. If that measurement is too long, multiply your long dimension by .75 and place the tape on the cover's short dimension instead.
3. Mark the tape 3:4 and draw a line across the page there when you're making  a sketch for 9x12, 12x16 or 18x24.
4. Do the same thing again, only multiply the SHORT DIMENSION by 1.25 or the LONG DIMENSION by .8
Mark that piece of tape 4:5 and use it to make sketches for 8x10, 11x14 or 16x20.
5. Remember to put tape guides on BOTH covers (pages turn both ways) and it may be necessary to have the 3:4 guide and the 4:5 guide taped to opposite edges of the cover. PIX BELOW: