A 1/4" is fine, you could even go a bit closer if you have a small sump. All you are attempting to do is determine the volume of oil needed to get to that level so you know how much to put in when everything is assembled. Remember it takes a quart to fill the filter, add another pint or so to fill the remote oil filter lines, add about 3/4 quart for wetting the engine, and a quart or so of oil for the oil in suspension and being whipped up by the rotating assembly. Assuming a six-quart sump that comes to a total of 8 1/4 quarts of oil. You could safely round down to 8 qts.
I highly recommend an oil accumulator.
Canton offers the original Accusump version and Moroso has an equally attractive offering they surprisingly call an Oil Accumulator. Don't use a wimpy 1 or 2-quart one, get the 3-quart one. The 3-quart one actually holds less than 2 quarts.
The big deal with the accumulators is twofold. The obvious first benefit is if your pickup is uncovered during acceleration, braking or turning the accumulator jumps in and supplies oil instead of air to the engine's oiling system. The not-so-obvious benefit is cold starts. Our cams get oiled last in the pecking order of oil deliveries. That means, if you didn't modify your primary (not secondary but primary) chain tensioners then the #1 cam journal on the front of each head gets the oil pinched out of it when the engine is shut off.
Later when you start the engine you have a cam-to-head contact patch that will scuff progressively more and more. One day the scuffing will be enough to seize the cam in the head and you will break a timing chain or chains along with a good number of other parts. The oil accumulator will prevent this by flooding the engine oil galleys with oil when you turn the ignition key and before the engine starts.
If you have the time, build your own pan that looks something like this;
Design it to clear or clearance K-member areas where it might interfere. There is a trap doored baffle in the middle of the pan that slams shut on braking and swings open on acceleration. The swinging pickup follows the oil as it goes from front to back and when I make a hard right turn and uncover the pickup the oil accumulator steps in to save the engine.