Co-ordinate system and Inch/pixel difficulties in VB

I have an app in development that gets input from a user and draws lines in VB6.
These lines describe a 3D round object within a blank round object the user also defines.
Problem is that inch values and pixel values are not compatible for use when I use the x2y2 points of a line to properly draw the next line using those values.
Example: user wants a 1 inch line. x1y1 values of the start point will be 0,0. x2y2 will be X0,Y1 "inch"
(Not really since 1 inch equals 0.99 something)
I need these values to be exact to 3 decimal places in order to properly draw a line at an angle to it using them as x1y1 of line 2 etc. The endpoint values must remain correct throughout the process. I'm already storing the results of the input as an array and am trying to keep it as simple as possible.
The drawing really needs to closely resemble what the user is defining and angles get screwed even worse since a line on a 30 degree angle will have more or less error than a line on some other angle or perpendicular line will.
Any suggestions?
Also, Microsofts X0,Y0 forces me to do gymnastics since X and Y are always negative. I need the first quadrant of a co-ordinate system, not the 4th. (braniacs)
thanks for considering this dilemma.
Tim Garrison