Schedule looks OK
Are sure the mark that you now call a scratch like mark is a crack? Could it be a mark from the shelf that was not eliminated by the second firing? Often a flip to put the bottom up will remove marks from the bottom. Of course, you must fire the top again to fire polish.
Do not apolgise for using the metric measurement system. All the world outside USA and some parts of Canada use it. Imperial measurements are the past. 
Stephen Richard
blogs at: http://www.verrier-glass.blogspot.com/ and http://www.glasstips.blogspot.com/
