armhole decreases

[HarryH]

Hi there - I have successfully begun to decrease by casting off the first 6 sts in consecutive rows etc. However, the pattern then said to decrease 1 st at the end of the next 3 rows, which I have also done. Then it says to dec 1 on foll 2 alternate rows then on following 4th row. I am confused on two details:

1 - if I decrease on every alternate row then i am always decreasing on the same armhole - won't this harm the symetry?

2 - the pattern says i should have 87 sts after the first 2 decrease rows (I do). It then says I should have 75 left after the final decrease. However, the above instructions look like they only decrease the sts to 81. Does this mean I should carry on decreasing every 4 rows until I do reach 75? If so I would be decreasing on the same side every time.

Help!!