Very Complex Equation Using TSQL Need Help

I am not sure if I am posting this is the right place, please let me know if a better place exists.

I am trying to figure out the distance from a point on the earth to a route between two other points. All points that I have are in latitude and longitude seconds

IE: 51-52-40.6700N => 186760.6700N

I am able to find the distance between two points on the globe using a custom function FindDist(latitude1, longitude1, latitude2, longitude2)
I did the legwork for the math and was able to come up with this psudo code:
 p1 = start of route
p2 = end of route
p3 = point of reference

p4 = INTERSECT(LINE(p1,p2), PERPENDICULAR_LINE(p1,p2,p3))
 
The direct distance between p4 and p3 is somewhere in the range of 0 - 70 and encompasses all points in the US (We only deal in the US) which is incorrect. If I do the conversion of p4 to latitude and longitude I get numbers that start out at 1000 which is also incorrect.

I am doing the conversion with the following:
 DECLARE @lat2 VARCHAR(25)
SET @lat2 = (@p4x * 3600)

IF @p4x < 0
BEGIN
SET @lat2 = @lat2 + 'S'
END

IF @p4x > 0
BEGIN
SET @lat2 = @lat2 + 'N'
END

DECLARE @lng2 VARCHAR(25)
SET @lng2 = @p4y * 3600

IF @p4y < 0
BEGIN
SET @lng2 = @lng2 + 'W'
END

IF @p4y > 0
BEGIN
SET @lng2 = @lng2 + 'E'
END 

I am not sure, however, how to either:
a) find the latitude / longitude of p4
b) find the distance in miles between p3 and p4

Either of these will solve my problem

I know this is a rather odd question, but if anyone can help me that would be great. If you need more information, let me know and I will try and explain things further.