PPM is based on weight, not volume. The oil has to be a weight, and the ZDDP has to be a weight. Quarts and ounces are just what we happen to pour it out in, not what it is specified in.
Weigh the booster. Turn the ZDDP in the booster into simple grams of "booster powder" (since you know the ppm) as if you had taken it all out magically. Now divide it by 12 to get the "booster powder" per Nyquil cap. Do the same math to the remaining weight of oil to get the weight per ounce of empty oil.
Weigh the bulk oil. Do the same magical removal from your bulk oil and state the total weight "booster powder" it contains and the total weight of the remaining bulk oil.
Add the Nyquil cap's worth of booster powder and the existing oil's booster powder, then reverse the math with the original weight of bulk oil (less the weight of the bulk oil booster powder) plus the weight of 1 ounce of booster oil (less the booster powder course).
The result should get close to reality.
Why do it this way? You have booster ppm stated against the empty booster oil weight and the bulk oil ppm stated against the empty oil weight and the math to do the related rates intermixing of the two systems is calculus and that is well beyond my old fart pay grade ....
But heck, even I can turn them back into "booster powder" add them together then apply them to the weight of the oil (minus the powder) plus the weight of an ounce of booster oil (minus the booster powder) and I should get pretty close to reality.
Or I could just follow the directions on the bottle .....