If I Understand your question well , all you need actually is modify the appearance of your formula (suppress or not suppress) not the calculation itself, so that at your last formula it should be like that
WhilePrintingRecords ;
shared numberVar totalsetPsum;
totalsetPsum-{sp_report_select_view_madeup_shipping_instruction_details;1.totalsetreg};
after that you will need to open format formula like that
right click the formula -> choose Format field -> from the format editor window that appears select Common tab -> beside Suppress CheckBox there's formula button that controls when to suppress it press it and add this line
if PageNumber = 1 then true else false
it should do the trick for you without affecting any calculation.
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