You might need to run custom code to calculate in flow, Attached a similar flow that will store current day sales in shop metafield. You can refer this and update as required
803ced5d0235f9d359f373518a1514773d61001ea222f2f02a41a12b25422d8c:{“__metadata”:{“version”:0.1},“root”:{“steps”:[{“step_id”:“a32a22b6-91a0-4490-b5c5-edee01717659”,“step_position”:[940,1360],“config_field_values”:,“task_id”:“shopify::admin::order_created”,“task_version”:“0.1”,“task_type”:“TRIGGER”,“description”:null,“note”:null,“name”:null},{“step_id”:“31b2c800-4b03-11ef-a5a4-6de6866cd29a”,“step_position”:[1320,1520],“config_field_values”:[{“config_field_id”:“input”,“value”:“query{\n shop{\n name\n ianaTimezone\n metafields{\n namespace\n key\n value\n }\n }\n order {\n createdAt\n totalPriceSet{\n presentmentMoney{\n amount\n }\n }\n }\n \n}”},{“config_field_id”:“script”,“value”:“export default function main(input) {\n // Find the metafield for today’s sales\n let todaySalesMetafield = input.shop.metafields.find(\n (metafield) => metafield.namespace === "custom" && metafield.key === "sale_day_date"\n );\n\n // Get the current date in the shop’s timezone\n const orderDate = new Date(input.order.createdAt);\n const currentDate = orderDate.toISOString().split(‘T’)[0]; // Format: YYYY-MM-DD\n\n // Initialize or reset todaySales\n let todaySales = 0;\n\n // If the metafield exists and its date matches the current date, use its value\n if (todaySalesMetafield && todaySalesMetafield.value) {\n const [storedDate, storedSales] = todaySalesMetafield.value.split(‘|’);\n if (storedDate === currentDate) {\n todaySales = parseInt(storedSales) || 0;\n }\n }\n\n // Add the current order amount to todaySales\n todaySales += parseFloat(input.order.totalPriceSet.presentmentMoney.amount);\n\n // Prepare the new value to be stored in the metafield\n const today_sales_config = ${currentDate}|${todaySales};\n\n return {\n today_sales_config,\n total_sales: todaySales,\n };\n}”},{“config_field_id”:“output_schema”,“value”:“"The output of Run Code"\ntype Output {\n "The message returned by the script"\n today_sales_config: String!\n total_sales:Int!\n \n}”}],“task_id”:“shopify::flow::run_code”,“task_version”:“0.1”,“task_type”:“ACTION”,“description”:null,“note”:null,“name”:“Run code”},{“step_id”:“b5d9a720-4b03-11ef-a5a4-6de6866cd29a”,“step_position”:[1720,1500],“config_field_values”:[{“config_field_id”:“namespace”,“value”:“custom”},{“config_field_id”:“key”,“value”:“today_sales”},{“config_field_id”:“value”,“value”:“{{runCode.total_sales}}”},{“config_field_id”:“value_type”,“value”:“single_line_text_field”}],“task_id”:“shopify::admin::add_shop_metafield”,“task_version”:“0.1”,“task_type”:“ACTION”,“description”:null,“note”:null,“name”:null},{“step_id”:“6ab7bf50-4bdc-11ef-9443-d7dcbd79048d”,“step_position”:[2080,1500],“config_field_values”:[{“config_field_id”:“namespace”,“value”:“custom”},{“config_field_id”:“key”,“value”:“today_sales_config”},{“config_field_id”:“value”,“value”:“{{runCode.today_sales_config}}”},{“config_field_id”:“value_type”,“value”:“single_line_text_field”}],“task_id”:“shopify::admin::add_shop_metafield”,“task_version”:“0.1”,“task_type”:“ACTION”,“description”:null,“note”:null,“name”:null}],“links”:[{“from_step_id”:“a32a22b6-91a0-4490-b5c5-edee01717659”,“from_port_id”:“output”,“to_step_id”:“31b2c800-4b03-11ef-a5a4-6de6866cd29a”,“to_port_id”:“input”},{“from_step_id”:“31b2c800-4b03-11ef-a5a4-6de6866cd29a”,“from_port_id”:“output”,“to_step_id”:“b5d9a720-4b03-11ef-a5a4-6de6866cd29a”,“to_port_id”:“input”},{“from_step_id”:“b5d9a720-4b03-11ef-a5a4-6de6866cd29a”,“from_port_id”:“output”,“to_step_id”:“6ab7bf50-4bdc-11ef-9443-d7dcbd79048d”,“to_port_id”:“input”}],“patched_fields”:,“workflow_name”:“Sum orders”}}