tuning your sigma lens. this is my advice.. sorry to hijack your thread but i am just trying to help.. those photos with the nikon lens really do look good.
that scale and ruler thing you got is great. but not once you need to start testing lenses where you need to get far away for tuning the infinity setting it was impossible on the target and ruler that i have (might work fine with a lens with that much zoom). personally i think this house number sign worked better than the target and ruler that i have with the 18-35mm art lens... i used the lens wide open or close to it on a tripod with a shutter speed of 1/500. i was trying to do it hand held and could not get it right. finally got out the tripod and could see what + or - number gave me the perfect focus. when the focus windows on the lens read right between 2 meters and infinity, a little closer to the infinity setting is how far back i chose to stand from the target and that seemed to work well.
up close something like that target and ruler is great but not when you need to start getting fa away to test infinity settings the text was just too small for the 18-35mm, what you need to tune has so much zoom that may not be a issue the target and ruller may work fine
if you do not have much time i would suggest doing 600mm one day or even part of 600 one day and part of it the next., do 150mm some other day, and so on and you will eventually get it right.. figure out if it is front or back focusing and take one photo for say +5 +10 +15 +20 say +10 looked the best but was not perfect. than go +6 +7 +8 +9 +10 +11 +12 +12 +14 and see which one of those is dead on. man it is a time sucker.. but its well worth it.
the first one is not cropped, the second one is cropped to see if it hit focus on the numbers. this was a great focus testing target, testing the focus at the closer focus settings i used a target with ruler and also i piece of paper taped to the wall with small text on it.
now that nikon lens looks to be excellent from everything i have seen so take your time, do a little tuning on the sigma here or there, write down what you have tried and what numbers gave you the best results. and you can get it done over time. when it was not focusing correctly the house numbers did not look very sharp. one the adjustment where they did look sharp is where i left it.