as my eyes get fuzzier with age I have found that enlarging the slot in the chipguard at the bottom about 1/8'' wider gives me much better view of the blade and cut line without sacrificing dust collection ability. I used a rotary tool and took off some plastic on each side till I got a good view. If I find a camera I'll post a pic.
also a note on the blades, there are four blades s75/4fs 486549, s75/4fsg 486551 s105/4fsg 486552, s145/4fsg 490256 that are made specifically for cutting at precise angles and square. they are thicker and resist flexing which gives them the ability to due this.
The s75/4fs is a hollow ground (tooth set in) which makes better strait cuts and finer finishes vrs the others are tooth set out and better for curves/radius work. let the blade do the work and don't force it on thicker woods with the longer blades for best results.
a helpful note when changing between blades is to check the blade guide set. it is adjustable using the allen in the base. if you go to a thicker blade and they are set to tight, the guides being carbide, will quickly heat up the blade and cause it to bend backwards. I've seen them turn red hot and fold back.( not good)and then conversely, if they are set to loose and you put in a thin blade they will allow the blade to flex and result in a poor cut. Set the guides to be snug but not allow binding, a little lite bluing on the blade is normal, but black is a tad tight.
not many people realize this fine feature on festool jigsaws, this is another thing I believe sets them apart from others. check it out next time you cut with it and it might help following the line even easier.
Bill