The short answer to your question on when the participant of the Holy Mass and you cite Novus Ordo make the sign of the cross is there are two types or classification.
There are certain part on the Holy Mass that it is "required" and there are others that are called "optional".
Your question falls on the so called "optional". That's why the Mass participants have freedom to exercise their God-given freewill according to one's disposition.
For someone who understand the meaning of every part of the Holy Mass and when to acknowledge the meaning or significance of that part, making the sign of the cross is an outward pious gestures of accepting that part of the Holy Mass.
Now, for some Catholic members who are not well-informed on the Value and Sanctity of the Holy Mass and what graces a faithful can derived out of participating in the Holy Sacrifice of the Mass, unfortunately we cannot expect all participants to be on the same level of dispositions.
In this view, whether the participants make or not make the sign of the cross in the part where it is consider optional it will not in any way lessen his participation.But a well-disposed participants certainly will receive more graces than the latter.
Roman Catholicism draws a distinction between liturgical and non-liturgical use of the sign of the cross.
The sign of the cross is required at certain points of the Mass:
1.the laity sign themselves during the introductory greeting of the service,
2.before the Gospel reading (small signs on forehead, lips, and heart),
3.and at the final blessing;
1.other times during the Mass when the laity often cross themselves are during a blessing with holy water,
2. when concluding the penitential rite,
3.immediately after receiving Communion,
4.and when concluding private prayer after Communion.
In the ordinary form of the Roman Rite the priest signs bread and wine once before the consecration. In the Tridentine Mass the priest signs the bread and wine 25 times during the Canon of the Mass, ten times before and fifteen times after they have been consecrated.
The priest also uses the sign of the cross when blessing a deacon before the deacon reads the Gospel, when sending an Extraordinary Minister of Holy Communion to take the Eucharist to the sick (after Communion, but before the end of the Mass), and when blessing the congregation at the conclusion of the Mass.
Ordained bishops, priests and deacons have more empowerment to bless objects and other people. While lay people may preside at certain blessings, the more a blessing is concerned with ecclesial or sacramental matters, the more it is reserved to clergy. Extraordinary ministers of Holy Communion do not ordinarily have a commission to bless in the name of the Church, as priests and deacons do. At this point in the liturgy, their specific function is to assist the clergy in the distribution of holy Communion. Extraordinary Ministers of Communion, blessing those who do not wish to or cannot receive communion can speak or raise the hand but not make the sign of the cross over the person.
A priest or deacon blesses an object or person with a single sign of the cross, but a bishop blesses with a triple sign of the cross. In the Catholic organization the Legion of Mary, members doing door-to-door parish surveys bless the homes of those not home by tracing the sign of the cross on the door
In closing, God the Father thru Jesus Christ founded and established the Church for the administration of the Sacraments and the Holy Spirit is guiding and sanctifying the action of the Church towards the salvation of all the redeemed.And The Catholic Church expect every members to participates in the Sacraments in manner worthy of receiving the Real Presence of Jesus Christ thru the Holy Eucharist and expect the highest reverence in receiving this powerful Sacraments.