Per WYeast:

REPITCHING BY WEIGHT OR VOLUME:

Estimates of cell counts can be made using percent yeast solids of the slurry. Percentage of yeast solids per volume of slurry can be estimated by allowing a sample to sediment under refrigeration and estimating the percent solids. Generally 40-60% yeast solids will correlate to 1.2 billion cells per mL. This will vary with the yeast strain. By using this method with every brew, a brewer can achieve consistent pitch rates batch to batch resulting in a more consistent product.

Once the brewer has determined the desired pitch rate and cell density of the slurry the brewer can collect the appropriate quantity of yeast. Slurry can be harvested based on volume or weight. 1.0 L (1 quart) of yeast slurry (40% yeast solids) weighs approximately 1.1 Kg (2.4 lbs). The following guidelines will deliver the appropriate pitch rates.

Ales with a specific gravity < 1.064 (16 °P): pitch 1.0 Kg (2.2 lbs) of thick slurry (40% yeast solids) per 1 BBL (1.17 hL) or 1 Liter (1 quart) of thick slurry per 1 BBL (1.17 hL)

Lagers with a specific gravity < 1.064 (16 °P): pitch 2.0 Kg (4.4 lbs.) of thick slurry (40% yeast solids) per 1 BBL (1.17 hL) or 2 Liters (2 quarts) of thick slurry per 1 BBL (1.17 hL)

High lagers with a specific gravity > 1.064 (16 °P): pitch 3.0 Kg (6.6 lbs.) of thick slurry (40% yeast solids) per 1 BBL (1.17 hL) or 3 Liters (3 quarts) of thick slurry per 1 BBL (1.17 hL)

Ref:

https://wyeastlab.com/resource/professional-yeast-harvesting-repitching/I believe 1 US BBL ≈ 31 US gal. 1 US Qt ≈ 32 US fl oz. So, ~ 1 fl oz per gal Ale and ~2 fl oz per gal Lager < 1.064. ~3 oz per gal > 1.064.

I use 12 fl oz pitch in 4.5 gal wort for Lager and 6 for an Ale (when I harvest and repitch). I want the desired yeast to outperform any undesirable microbes I may have inadvertently harvested or otherwise introduced. I do this as described in post #9 above.

*Disclaimer*: Any comment I add is simply the way I brew beer. I am not paid or sponsored by anyone. There are certainly other ways that can be equally effective which other brewers may contribute. This is what I’ve found that works for me using my equipment and processes so I offer this for your consideration. YMMV