Hello, Kathy -Doesn't the tissue pattern for the garment you wish to sew have seam markings on it, indicating the seam allowance? See image 02 here:
https://www.thesprucecrafts.com/techniques-for-sewing-clothing-2978471It shows an edge of a pattern piece with the seam allowance printed on it.It has been a long time since have sewn garments, but I recall that unless the pattern indicates otherwise, seams are sewn at 5/8 inches. If the pattern indicates 1 inch seam allowances and no other seam allowance depth, then, a 1 inch seam allowance it is.I do not have Simplicity Pattern #1800 myself. If you do not have the printed sewing instructions for the pattern (where the seam depth may be stated), then you may wish to contact Simplicity Creative Group and ask to purchase replacement instructions for your pattern.This is the US toll free phone number to call:833-201-5260To send an email, this is the link to the web page with a form you can use to fill out to ask about the availability of replacement instructions:
https://www.simplicity.com/contactus/?topic=OTHERBest wishes.,