You don't need a mod, you can use basic trigonometry to determine range visually.
This is the theory behind the Stadimeter, namely that the visual height of a ship in the scope is a function of the range. The horizontal bars in the scope are there to allow you to calculate this.
I use this technique regularly in SH5, with Arclight's periscope mod, the formula at hi-power (6x) is:
mast height in meters x 200 / horizontal bars in scope = range in meters
I don't use this in SH4 since I rely on Nisgeis' Radar mod, but through tests, you should be able to determine the formula. For this to work, you need a recognition manual which gives you correct height to the reference point you use. SCAF or OTC should have this.
This technique was used by both German and U.S. skippers during WW2, both as an alternative and a double check on the Stadimeter.
Quote:
Officer knows or can estimate the masthead height of the target in feet the number of horizontal divisions covered by the ship between its water line and masthead will be a measure of the range of the target. In the figure the target subtends 5 divisions in high power and 1 1/4 divisions in low power. It would obviously be impracticable to convert this value of angle to range at each periscope observation. The obvious solution is some form of precomputed graph or scale. We know that at a range of 1000 yards, 17 1/2 yards, or 52.5 feet will subtend an angle of 1 degree. Using this relation we can deduce the following formulas:
R(range) = (19.1 h / n) R(range) = (76.2 h / N)
R = range in yards
h = height in feet
n number scale divisions low power
N number scale divisions hi power
Plate III is a picture of one type of scale ("range omnimeter") which may be constructed. In the figure the masthead height of the target is 100 feet. The arrow of the sliding scale is set opposite the masthead height and the range is read opposite the number of divisions. In this case 1550 yards is read opposite 5 divisions high power and 1 1/4 divisions low power. Ranges obtained in this manner are commonly referred to as "telemeter ranges". Estimates of ranges should be made to the nearest 1/8 division.
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http://hnsa.org/doc/attack/index.htm (chapter 5-2)