a rule question

I have two tables one is called "inventory" and the other is
"inventory_usage". When I insert into inventory_usage info like number of
widgets used for a given order_number I would the in_stock entry in the
inventory to be adjusted. Here are the two tables and my current attempts at
the rule:

create sequence "inventory_id_seq" start 1 increment 1;
create table "inventory" (
"inventory_id" int4 default nextval('inventory_id_seq') NOT NULL
unique,
"in_stock" int4 default 0,
"name" character varying not null,
"description" character varying not null,
"cost_per_unit" money,
"vendor_code" character varying,
"vendor_name" character varying,
"vendor_address_id" int4 references address (address_id),
primary key ("name")
);
grant update on inventory to flip;
grant select on inventory to flip;

create sequence "inventory_usage_id_seq" start 1 increment 1;
create table "inventory_usage" (
"inventory_usage_id" int4 default nextval('inventory_usage_id_seq')
NOT NULL unique,
"inventory_id" int4 references inventory (inventory_id),
"timestamp" timestamp default now(),
"used" int4 not null,
"printer" character varying,
"real_lpi" real,
"order_num" int4,
primary key ("inventory_usage_id")
);
grant all on inventory_usage to flip;
grant all on inventory_usage_id_seq to flip;

create rule inventory_usage_insert_rule as
on insert to inventory_usage
do
update inventory set inventory.in_stock=(select in_stock from inventory
where
inventory_usage.inventory_id=inventory.inventory_id)-inventory_usage.used
where inventory.inventory_id=inventory_usage.inventory_id;

I have also tried:

create rule inventory_usage_insert_rule as
on insert to inventory_usage
do
update inventory set new.in_stock=old.in_stock-inventory_usage.used
where inventory.inventory_id=inventory_usage.inventory_id;

I think the second one would work if I put a in_stock column in the
inventory_usage table. But this does not seam like a slick solution.

Thank you
Brad Paul