Hi Yair,
Thank you for reaching out. When you generate forecast metrics using KeywordPlanService, use the account which is related to the campaign. Don’t forecast from an unrelated account as things like quality score and creative content are factored into the forecast. So in your case, do not use the loginCustomerId as the customerId. Let me know if you have any further questions.
Thanks and regards,
Xiaoming, Google Ads API Team