Adjusting the Shimano 105 5800 front derailleur can be a bit difficult. Especially for the low adjustment with the front derailleur on the smallest chainring and the rear derailleur on the biggest cog. It is not always possible to get the complete clearance of 0 – 0.5 mm between the skid plate of the chain guide and the chain. It looks like the new skid plate is there for a reason.
Besides the low and top position the 105 5800 front deraileur also has 2 trim positions. The low-trim and top-trim position. The top-trim position is used for adjusting the cable tension but can also be used to ride longer on the largest chainring before switching to the smallest chainring. When you are cycling on the largest chainring swith the front gear one click with the smallest peddle. The chain will not yet swith to the smaller chainring but you will get more space between the chain and the inner plate of the chain guide. Then you can ride a bit longer on the larger chainring before switching to the smaller one. This position is called the top-trim position.
Also found a nice video about the 105 front derailleur adjustment on youtube: Front Derailleur Trim Set Up & Adjustment
