Using a portion of a starter

If a 1L yeast starter on stir plate gives >50% too much yeast than needed for pitching, do the following steps make sense?:

1. Measure the weight of the empty flask.
2. Make a 1L starter on a stir plate, ferment to completion, chill & decant.
3. Weigh the flask and contents. Subtract the original weight. Ratio the amount of yeast needed to the expected yeast in the starter to determine the final weight of starter to leave behind in the flask + empty flask weight.
4. Shake to mix contents up VERY well and pour some into wort, weigh flask + remaining contents and keep doing this until what's left is close to the amount in step 3.

This is for a 3 gallon ale that should get more yeast than a single pack but less than a stirred starter. Stirred 1L starters seem to go well and I prefer to keep making these even if I don't need that much yeast growth. I may try to get twice as much yeast as needed to store for a second batch.

I don't see a need to incorporate the ratio. I would just use the volume of the container and estimate. Maybe I am not very scientific.

If using the scale, I would reduce the weight of the filled container in grams by the number of millileters desired in the wort. The density of the starter is similar to water. 1 gram being 1 mililiter.

In general, your process seems sound. The most important part being that the yeast are mixed and therefore evenly distributed in solution.

Just lop it in half by volume. Growing more yeast than you need and using it for future batches works great.