It doesn't matter where Yoshi is standing. The line gets drawn depending on the angle the game is tilted. In both screenshots above, the angle is 0. Please note I'm not very good with math things. I achieve much of what I want through trial and error.
-- draw line from yoshi to this destination (*80 factor is how far the line goes)
des1 = math.sin(AngleDegree*RPD)*-80
des2 = math.cos(-AngleDegree*RPD)*80
des3 = math.sin((AngleDegree+90)*RPD)*-80
des4 = math.cos(-(AngleDegree+90)*RPD)*80
gui.drawLine(Xscreen,Yscreen,Xscreen+des1,Yscreen+des2,0xFFFFFFFF) -- below yoshi
gui.drawLine(Xscreen,Yscreen,Xscreen-des1,Yscreen-des2,0xFFFFFFFF) -- above yoshi
gui.drawLine(Xscreen,Yscreen,Xscreen+des3,Yscreen+des4,0xFFFFFFFF) -- left of yoshi
gui.drawLine(Xscreen,Yscreen,Xscreen-des3,Yscreen-des4+1,0xFFFFFFFF) -- right of yoshi (this is the line that looks off, but by adding +1 it looks as before)
I added +1 and it looks like before. This is good enough for my purposes.