To calculate how long your stay will be, you need to know what length is occupied by the tensioning system. Are you doing deadeyes, or deadeyes with turnbuckles? What's the difference?
If you are going to use deadeyes only, you will need a toggle which is sized appropriately to the chainplate clevis pin hole. The lower thimble of the deadeye needs to be small enough to fit between the jaws of the toggle. The top thimble needs to be large enough to serve as a turning block for the lashings. For stays under more load, a larger thimble is needed as it will allow more lashings to lay next to each other without overlapping any of them. For stays under less load, smaller thimbles can be used as less mechanical advantage will be needed to tension the stays.
When you use turnbuckles, they become the main choice for tensioning the stays. The deadeyes and lashings are simply to absorb the constructional stretch and creep over the lifetime of the stay. Turnbuckles only have a few inches of adjustment, where deadeyes and lashings can absorb feet of stretch. Since the deadeye doesn't need to form a block and tackle system to tension the stays, the thimbles can be significantly smaller. In these cases, the thimbles are sized based on the rope used to make the deadeye rather than calculating the number of passes the lashing will need to make to create the desired mechanical advantage.
The turnbuckle attaches to the chainplate via a toggle jaw on the lower screw, and to the lower thimble of the deadeye via a toggle jaw on the upper screw. While turnbuckles are expensive, they do save the cost of purchasing an additional toggle.
The last difference between the two systems is the length added to the lower end of the stay. A deadeye will stretch out to around 12 inches in length, and the lashings should be between 18 to 24 inches in length. This will give you plenty of leeway for additional stretch or creep in the stay over the years without the risk of the stay and deadeye becoming two-blocked. If the deadeye were to become two-blocked, the splice would need to be opened up and relocated further up the stay.
A turnbuckle will add additional distance to the lower components of the stay, meaning that the stay should be cut even shorter. A 1/2 inch turnbuckle will be around 15 inches long when fully unscrewed, add to that a 12 inch long deadeye and 18 inches of lashings, and you now have 45 inches of the lower section composed of tensioning equipment.
In our example the stay would need to be cut 45 inches shorter for a setup of deadeyes and turnbuckles, or only 30 inches shorter for a setup of just deadeyes.
If you are unsure about deadeyes and would like to try them out but want to keep the door open to convert to turnbuckles in the future, cutting the stays to the length needed for a deadeye and turnbuckle, that way turnbuckles can be easily retrofitted should the desire arise.
One last point about positioning the eye splice in the lower portion of the stay is to consider damage from chafe. If your sheets will rub on the shrouds, consider positioning the end of the stay just higher than the area of chafe. This may result in a very long lashing which will still work fine and not cause the standing rigging to suffer.
The reason for setting the lashing in a high chafe region instead of the stay is the ease of replacement. The stay is going to chafe if a sheet is constantly rubbing on it, this is unavoidable. By positioning the eye splice higher than the area of chafe, the lashing will become the sacrificial piece that can be replaced when it becomes severely chafed. Lashings are much cheaper and easier to replace than a stay that has been spliced to exact size constraints.
If the chafe were to occur on the lower portion of the stay, the whole stay would need to be replaced. If the lashing were to become severely chafed, just the inexpensive lashing needs to be replaced.
Basically, deadeyes alone will have a slightly longer stay, where deadeyes and turnbuckles will have a shorter stay. The only modifications to this rule would be to protect the stay from chafing damage, where it might be set even shorter to remove the lower portion of the stay from risk of chafing damage.