First reading takes place as written in BWG.
Practicals as written:
Casting the spell is at the Ob of spell + Perception aptitude - Number of previous times cast as part of practicals.
When you have cast the spell a number of times equal to the aptitude you stop practicals and move onto Second reading. You may not cast the spell again until the second reading is done.
You cast the spell at the Ob of the spell + half the Perception Aptitude rounded up (or a minimum of +1 Ob).
When you have cast the spell a number of times equal to the aptitude you stop counting may choose to move onto Second reading. You may cast the spell at the above obstacle until second reading is done.
Second reading takes place as written in BWG.
This change allows a spellcaster to cast a spell with an obstacle penalty after practicals are usually done. The rules as written stop the spellcaster from trying to cast the spell without explaining the removal of this option in the fiction of the game.
I believe this is closer to how it worked in BWR where prior to second reading there was a double obstacle penalty.